stand-alone containment analysis of the phébus fpt tests with...

26
Research Article Stand-Alone Containment Analysis of the Phébus FPT Tests with the ASTEC and the MELCOR Codes: The FPT-0 Test Bruno Gonfiotti and Sandro Paci Dipartimento di Ingegneria Civile e Industriale (DICI), University of Pisa, Largo Lucio Lazzarino 2, 56122 Pisa, Italy Correspondence should be addressed to Bruno Gonfiotti; bruno.gonfi[email protected] Received 13 May 2017; Accepted 21 August 2017; Published 5 December 2017 Academic Editor: Tomasz Kozlowski Copyright © 2017 Bruno Gonfiotti and Sandro Paci. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. e integral Ph´ ebus tests were probably one of the most important experimental campaigns performed to investigate the progression of severe accidents in light water reactors. In these tests, the degradation of a PWR fuel bundle was investigated employing different control rod materials and burn-up levels in strongly or weakly oxidizing conditions. From the results of such tests, numerical codes such as ASTEC and MELCOR have been developed to describe the evolution of a severe accident. Aſter the termination of the experimental Ph´ ebus campaign, these two codes were furthermore expanded. erefore, the aim of the present work is to reanalyze the first Ph´ ebus test (FPT-0) employing the updated ASTEC and MELCOR versions to ensure that the new improvements introduced in such codes allow also a better prediction of these Ph´ ebus tests. e analysis focuses on the stand-alone containment aspects of this test, and the paper summarizes the main thermal-hydraulic results and presents different sensitivity analyses carried out on the aerosols and fission products behavior. is paper is part of a series of publications covering the four executed Ph´ ebus tests employing a solid PWR fuel bundle: FPT-0, FPT-1, FPT-2, and FPT-3. 1. Introduction In the last 40 years, several efforts have been carried out to investigate the release of fission products (FPs) from a damaged nuclear core and their subsequent behavior in the primary circuit and the containment system of a nuclear power plant during a severe accident (SA) sequence [1, 2]. Different experimental campaigns were carried out investi- gating specific phenomena or the coupling among two or more phenomena, although only the international Ph´ ebus experimental program was devoted to the study of a SA in its different phases and aspects, the core degradation, the trans- port of FPs, and structural materials through the primary system, and finally their release into the containment [3]. is experimental campaign was carried out in the integral Ph´ ebus facility at the “Institut de Radioprotection et de Sˆ uret´ e Nucl´ eaire” (IRSN) laboratories in Cadarache (F). Five integral tests were performed, investigating the main processes affect- ing the severe degradation of the nuclear fuel and control rods, the release of FPs and control materials, their transport through the reactor coolant circuit, and their deposition as aerosols in the containment vessel [4–6]. Different fuel burn-up levels, control rod materials, and thermal-hydraulic conditions were investigated [7]. e Ph´ ebus research pro- gram has provided a comprehensive database to improve the understanding of the various phenomena leading to fission product behavior. Furthermore, these experiments are the most representative integral cases, in such a way that this program has formed a valuable validation database for several severe accident codes [8]. is FPT-0 test employs, in detail, a bundle of fresh nuclear fuel with an Ag-In-Cd control rod, under steam-rich conditions [9]. In the present paper, a stand-alone containment analysis of this test is presented, employing the latest release versions of two of the most world- wide employed SA codes: ASTEC v.2.0 revision 3 patch 3 and MELCOR V2.1.6840. Both these integral codes are able to simulate a SA from the initiating event until the release of FPs outside the containment. e aim of the work is to investigate the main parameters influencing the FPs behavior inside the containment during the four main phases of the test (degradation, aerosol, washing, and chemistry phases). Different previous analyses (for the full-plant or the stand- alone containment) have employed too complex or too coarse Hindawi Science and Technology of Nuclear Installations Volume 2017, Article ID 1450648, 25 pages https://doi.org/10.1155/2017/1450648

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Page 1: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Research ArticleStand-Alone Containment Analysis of the Pheacutebus FPT Tests withthe ASTEC and the MELCOR Codes The FPT-0 Test

Bruno Gonfiotti and Sandro Paci

Dipartimento di Ingegneria Civile e Industriale (DICI) University of Pisa Largo Lucio Lazzarino 2 56122 Pisa Italy

Correspondence should be addressed to Bruno Gonfiotti brunogonfiottiforunipiit

Received 13 May 2017 Accepted 21 August 2017 Published 5 December 2017

Academic Editor Tomasz Kozlowski

Copyright copy 2017 Bruno Gonfiotti and Sandro Paci This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

The integral Phebus tests were probably one of the most important experimental campaigns performed to investigate theprogression of severe accidents in light water reactors In these tests the degradation of a PWR fuel bundle was investigatedemploying different control rod materials and burn-up levels in strongly or weakly oxidizing conditions From the results of suchtests numerical codes such as ASTEC and MELCOR have been developed to describe the evolution of a severe accident After thetermination of the experimental Phebus campaign these two codes were furthermore expanded Therefore the aim of the presentwork is to reanalyze the first Phebus test (FPT-0) employing the updated ASTEC and MELCOR versions to ensure that the newimprovements introduced in such codes allow also a better prediction of these Phebus testsThe analysis focuses on the stand-alonecontainment aspects of this test and the paper summarizes the main thermal-hydraulic results and presents different sensitivityanalyses carried out on the aerosols and fission products behavior This paper is part of a series of publications covering the fourexecuted Phebus tests employing a solid PWR fuel bundle FPT-0 FPT-1 FPT-2 and FPT-3

1 Introduction

In the last 40 years several efforts have been carried outto investigate the release of fission products (FPs) from adamaged nuclear core and their subsequent behavior in theprimary circuit and the containment system of a nuclearpower plant during a severe accident (SA) sequence [1 2]Different experimental campaigns were carried out investi-gating specific phenomena or the coupling among two ormore phenomena although only the international Phebusexperimental program was devoted to the study of a SA in itsdifferent phases and aspects the core degradation the trans-port of FPs and structural materials through the primarysystem and finally their release into the containment [3]This experimental campaign was carried out in the integralPhebus facility at the ldquoInstitut de Radioprotection et de SureteNucleairerdquo (IRSN) laboratories in Cadarache (F) Five integraltests were performed investigating themain processes affect-ing the severe degradation of the nuclear fuel and controlrods the release of FPs and control materials their transportthrough the reactor coolant circuit and their depositionas aerosols in the containment vessel [4ndash6] Different fuel

burn-up levels control rod materials and thermal-hydraulicconditions were investigated [7] The Phebus research pro-gram has provided a comprehensive database to improve theunderstanding of the various phenomena leading to fissionproduct behavior Furthermore these experiments are themost representative integral cases in such a way that thisprogramhas formed a valuable validation database for severalsevere accident codes [8] This FPT-0 test employs in detaila bundle of fresh nuclear fuel with an Ag-In-Cd controlrod under steam-rich conditions [9] In the present papera stand-alone containment analysis of this test is presentedemploying the latest release versions of two of themostworld-wide employed SA codes ASTEC v20 revision 3 patch 3and MELCOR V216840 Both these integral codes are ableto simulate a SA from the initiating event until the releaseof FPs outside the containment The aim of the work is toinvestigate the main parameters influencing the FPs behaviorinside the containment during the four main phases of thetest (degradation aerosol washing and chemistry phases)Different previous analyses (for the full-plant or the stand-alone containment) have employed too complex or too coarse

HindawiScience and Technology of Nuclear InstallationsVolume 2017 Article ID 1450648 25 pageshttpsdoiorg10115520171450648

2 Science and Technology of Nuclear Installations

Model ofreactor core

Verticalline

Testassembly

core

Experimental cell

Horizontalline (hot leg)

700∘C 150∘C

Steam generatorU-tube

Horizontalline (cold leg)

(10G3)

Hot leg Cold leg

Model of primary circuitincluding steam generator

Sump

Paintedcoupon

Condensingsurfaces

Dry surfaces

Paintedcondensers

Paintedliners

Sump

Model of reactorcontainment

Fission Product Caisson

Ph bus containmentPh bus reactor

Ph bus reactor

Figure 1 Schematic overview of the Phebus facility (original image taken from [3])

nodalizations [10ndash15] not able to catch the main thermal-hydraulics or aerosol phenomena or to be employed forfull-plant analysis For this purpose three different spatialnodalizations of the Phebus containment vessel have beendeveloped to show that too simple nodalizations are notable to simulate the thermal-hydraulic transientThe simplestmodel consists in 7 control volumes (CVs) 12 heat structures(HSs) and 7 flow paths (FLPs) while the most complex oneconsists in 21 CVs 33 HSs and 32 FLPs These three nodal-izations have been kept as simple as possible (as the numberof CVs and HSs) to allow a similar spatial schematizationalso for full plant analysisThese nodalizations have been alsodeveloped in the most identical way possible for both codesbut the differentmodelling approaches of certain aspectswereall exploited to obtain the best possible results with eachcode The present paper is a part of a series of publicationscovering the 4 Phebus tests executed with a solid PWR fuelbundle FPT-0 FPT-1 FPT-2 and FPT-3 The aim is to writea detailed paper for each Phebus test to underline and discussthe main aspects of the containment behavior in the differenttestrsquos conditions For this reason these papers will follow asimilar strategy in the closest possible way An additionalpaper is also planned to summarize themain findings of thesefour works providing also a comparison on the influenceof each specific sensitivity parameters investigated on eachspecific test Of these five papers the one covering the FPT-1 test has been already published [16] Observations madeduring these analyses led to a thorough understanding of thein-containment source term and its coupling with thermal-hydraulics under unsaturated but condensing atmospheres

2 The Pheacutebus Facility

The Phebus facility is downscaled to a size of 5000 1 ofa typical French 900 MWe class pressurized water reactor(PWR) A schematic sketch of the facility is shown in Figure 1[3] The first component is the driver core and its coolingcircuit encapsulated inside a cylindrical shroud The coreconsists of a PWR fuel bundle two instrumented fuel rodsand a control rod Different fuel burnup levels and controlrod materials were investigated in each Phebus test Duringthese tests the driver core is heated up and irradiated torecreate the temperature increase supposed to occur duringa loss of coolant accident (LOCA) and the FPs build-upduring normal operations At the bottom of the shroud a lineis installed to inject steam Different steam injection slopeswere investigated in each test to analyze the fuel behaviorunder strongly or weakly oxidizing conditions At the topof the shroud the experimental line (EL) transports steamincondensable gases and FPs into the containmentThe EL issubdivided into three parts

(i) the first part made of Inconel-600 simulates a PWRhot leg

(ii) the second part still made of Inconel-600 simulatesa U-tube type SG

(iii) the third part made of AISI 304L simulates a PWRcold leg

Finally the EL enters into the center of the containment vesselto simulate a cold leg LOCA

Science and Technology of Nuclear Installations 3

The containment vessel has a total free volume of 10m3with a height of about 5m and an inner diameter of about18m The containment has a cylindrical shape with a watersump in the lower part The sump has a height of 06m andan inner diameter of 0584m to reproduce a representativeatmosphere water exchange surface [17] The top and thebottom parts of the vessel are closed with a semi-ellipsoidalstructure The containment walls are made of AISI 316L andtheir temperatures are controlled through two independentsystems respectively for the sump zone and for the ldquocylindri-calrdquo zone The aims of the two cooling loops are to decouplethe sumpwater temperature from the gas temperature and toavoid condensation onto containment walls A spray systemis also inserted in the lower containment zone to washdownthe FPs settled on the bottom vessel surfaces The sprayalmost covers the entire flow-through area of the cylindricalcontainment part and it is fed only by the water contained inthe sump

Three condensers are attached at the top vault to simulatethe cold structures of a reactor building The main aim ofthese condensers is to allow the control of the heat transferand steam condensation Each condenser has an externaldiameter of 015m a height of 25m and it is subdivided intotwo parts the ldquodryrdquo part of 0782m and the ldquowetrdquo part of1718m The total surface area of condensers is about 35m224m2 for the wet part and 11m2 for the dry part The innertemperatures of the two condenser halves are controlled bytwo independent loops to allow condensation only onto thewet part This wet part is also covered with an epoxy paintto allow the organic iodine formation Further details on thefacility can be found in the final report of the FPT-0 test [9]

3 The FPT-0 Test and Boundary Conditions

The Phebus FPT-0 test can be subdivided into four differentphases

(i) The degradation phase when the driver core is heatedup to allow the progressive melting of the fuel and thesubsequent release of FPs This phase starts at 0 s andends at 18138 s The fuel employed consists in a freshPWR rod bundle irradiated for a very short period toallow the formation of short-living FPs with a Ag-In-Cd control rod The degradation occurs in a stronglyoxidizing atmosphere

(ii) The aerosol phase with the containment maintainedin stable conditions and isolated from the driver core(at 20438 s) This phase starts at 20438 s and ends at86018 s

(iii) The washing phase with the activation of the sprayinside the containment to wash down the FPs settledonto the elliptic bottom The FPs are then collectedinside the sumpThis phase starts at 111218 s and endsat 112118 s

(iv) The chemistry phase During this last phase nocondensation occurs and the iodine chemistry in thesump is analyzed This phase starts at 133718 s andends at 433848 s that is till the termination of thetest

01

02

03

04

05

06

07

017

018

019

02

021

022

023

0 5000 10000 15000 20000 25000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 2 Total pressure and rh evolutions during the first 25000 sof the test

0

005

01

015

02

025

0

05

1

15

2

25

3

35

0 5000 10000 15000 20000 25000

Stea

m m

ass fl

ow ra

te (g

s)

Time (s)

Steam injection

(2

mas

s flow

rate

(gs

)

(2 injection

Figure 3 Steam and H2injection flow rates during the test

In Figure 2 the containment total pressure and the relativehumidity (rh) trends during the first 25000 s of the FPT-0 test are shown The data provided inside the FPT-0 finalreport [9] do not cover the entire test and for the rh scarcedata points are also reported During the first 25000 s thepressure and the rh follow the evolution of the steam flowrate (Figure 3) After the termination of the steam injec-tion the pressure (Figure 4) remains almost constant until40000 s when the wet condensersrsquo temperature is increasedto 38315 K This increase lasts until the beginning of thewashing phase preparation activities (88358 s) Such actionsconsist in an initial decrease of the vessel cylindrical walltemperature followed by a subsequent increase the decreaseof the wet condensers and the sump wall temperaturesand the isolation of the vessel elliptic bottom from theprimary organic loops controlling the vessel cylindrical walltemperature (Figure 5)The primary organic loops are closedcircuits in which an organic coolant flows These loopsare equipped with heaters and pumps to set the vesselrsquoswalls temperatures according to the test schedule The sprayactivation triggers an abrupt containment total pressure and

4 Science and Technology of Nuclear Installations

01

02

03

04

05

06

07

011

013

015

017

019

021

023

0 50000 100000 150000 200000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 4 Total pressure and rh evolutions up to 20000 s after thebeginning of the test

300

320

340

360

380

400

420

0 50000 100000 150000 200000

Wal

l tem

pera

ture

(K)

Time (s)

SumpBottomVessel

Dry condenserWet condenser

Figure 5 Wall temperature evolutions during the first 200000 s ofthe test

rh decrease After the termination of the washing phasethe elliptic bottom is reconnected to the primary organicloop and the wet condensers and the sumpwall temperaturesstart to increase again for the following chemistry phase Thewall temperatures were all employed as boundary conditionsNo experimental data for the total pressure and the wallrsquostemperatures were provided in [9] after 180000 s During thislast phase (chemistry phase) only the rh was measured forthree times at 223718 s at 310118 s and at 396518 s A rhvalue of 30 was reported for the first two measurementswhile a value of 31 was shown for the third one

Twomain injections occur in the containment during thetest the steam and the H

2injections The steam is injected at

the bottom of the degraded fuel element but only a fractionflows in the containment due to the steam reaction with thefuel clad H

2is produced by the Zircaloy oxidation reaction

but in the present paper such phenomenon is not simulatedso the H

2mass flow rate is imposed as a boundary condition

as well as the steam flow rate into the containment (Figure 3)During the test several samplings are also performed toanalyze the composition of the containment atmosphere

375376377378379380381382383384385

0 10000 20000 30000 40000

Atm

osph

eric

tem

pera

ture

(K)

Time (s)

402 m30 m

232m1351m

Figure 6 Atmospheric temperatures at different containmentheights during the fire first 40000 s of the test

365

370

375

380

385

390

395

400

405

0 50000 100000 150000 200000Time (s)

Atm

osph

eric

tem

pera

ture

(K)

232m

Figure 7 Atmospheric temperature evolution at 232m up to200000 s

Figure 6 shows the mean atmospheric temperature atdifferent containment heights until 40000 s while in Figure 7the atmospheric temperature at 232m only until 200000 sis reported because no data were provided for the otherheights In Figure 8 the sumpwater PH evolution for the first200000 s of the test is reported Again the data provided forthe PH evolution do not cover the entire test In Figure 9 thetotal condensation rate on the wet condensers until 35000 sis shown (no condensation occurs after that time) Finallythe atmospheric and the sump dose ratesrsquo evolutions upto 200000 s are reported in Figure 10 After 180000 s thedose rates in the atmosphere and the sump were measuredonly at 277338 s and values of 0245Gys (atmosphere) and0127Gys (sump)were reported Amore detailed descriptionof the test can be found in the FPT-0 final report [9]

4 Codes Employed

As previously said two codes have been employed for thepresent paper ASTEC v20 revision 3 patch 3 and MELCOR

Science and Technology of Nuclear Installations 5

4

42

44

46

48

5

52

54

0 50000 100000 150000 200000Time (s)

PH

Sum

p pH

(mdash)

Figure 8 Sump PH evolution for the first 200000 s of the test

0

05

1

15

2

25

3

0 10000 20000 30000

Con

dens

atio

n ra

te (g

s)

Time (s)

Condensation rate

Figure 9 Total condensation rate during the first 40000 s of thetest

V216840 Both codes are capable of simulating a SA from theinitiating event to the release of FPs outside the containmentThey are based on a lumped-parameter approach where thespatial domain is subdivided into CVs connected throughFLPs Inside each CV a nonequilibrium condition is appliedfor the liquid and gaseous zones The circulation among thedifferent CVs is provided by the FLPs connecting them Asimplified momentum balance equation is introduced foreach FLP but still capable of catching the main circulationphenomena inside the primary system or the containmentBoth codes include also additional packagesmodules to cou-ple thermal-hydraulics with aerosolFP calculations Signifi-cant differences exist regarding the aerosol and FP treatmentin terms of models numerical approach and coupling withthermal-hydraulics but such differences will not be discussedin the present paper

ASTEC is jointly developed by IRSN (France) and GRS(Germany) The code is subdivided into different modulesbut in the present work only the CPA IODE and SYSINTmodules were employed The CPA module deals with thecontainment thermal-hydraulics and aerosol behavior andit is subdivided into three submodules THY for thermal-hydraulic analysis AFP for FP behavior and FIPHOST for FP

0 50000 100000 150000 200000

Dos

e rat

e (G

ys)

Time (s)

Atmospheric doseWater dose

1E + 02

1E + 01

1E + 00

1E minus 01

1E minus 02

1E minus 03

Figure 10 Atmospheric andwater dose rates in containment duringthe test

transport in containment [18] The IODE module deals withthe Iodine and Ruthenium chemistry in containment [19]Finally the SYSINT module manages safety systems basedon user inputs or plant conditions [20] Specific informationon the different models implemented inside each ASTECmodule can be found in the code manuals

MELCOR is developed by Sandia National Laboratory(USA)The code is subdivided into different packages but inthe present work only the control function (CF) the controlvolume hydrodynamic (CVH) the executive (EXEC) theflow path (FL) the heat structure (HS) the material proper-ties (MP) the noncondensible gas (NCG) the radionuclide(RN) the containment spray (SPR) and the tabular function(TF) packages were employed The CF and TF packages dealwith the tabular (TF) and control (CF) functions needed forthe correct activation of the other modules The EXEC pack-age manages the execution parameters of the simulation astime step beginning and ending simulation time output filesand so on The CVH and FL packages model the thermal-hydraulic behavior of coolant and incondensable gases TheHS package models the structures attached to the differentCVs The MP and NCG packages activate the structuralmaterials (MP) and the incondensable gases (NCG) neededfor the correct execution of the simulationsThe SPR packagemanages the sprays in the containment building Finally theRN package models the behavior of aerosols and vapors inthe whole plant from the release from the damaged fuelto the deposition on containment surfaces This packageincludes also a specific model to simulate the Iodine behaviorin gaseous and liquid phases Specific information on thedifferent models implemented in eachMELCORmodule canbe found in the code manuals [21]

5 Employed Nodalizations andBoundary Conditions

Three spatial nodalizations have been developed to simulatethe containment behavior during the FPT-0 test In Figure 11a sketch of the three spatial models is shown and inTables 1 2 and 3 the geometrical features and the initial

6 Science and Technology of Nuclear Installations

Table 1 HSrsquos characteristic lengths

Wall Characteristic length [m]Model 1

WSU 0146WB1 0416W3 2299W4 1007W2T 0146W4T 0416WET 015DRY 015

Model 2WSU 0146WB1 0416W7 0759W8 0759W9 0782W10 0504W11 0504W6T 0146W12T 0416WET4 015WET5 015WET6 015DRY 015

Model 3WSU 0146WB1 0265WB2 0142W13 0759W14 0759W15 0782W16 0504W17 0504W6T 0146W12T 0265W18T 0142WET4 015WET5 015WET6 015DRY 015

boundary conditions employed for these three models arereported [9] Identical total pressures (195 kPa) and rh (49)were employed for the three different nodalizations Theatmospheric and the sump temperatures of each CV were setaccording to the data coming from the sensors installed atdifferent vessel heights and the wall temperatures were setaccording to the boundary conditions shown in Figure 5

The first scheme (M1) has been employed to show thatnodalizations with few nodes are not capable of simulatingthe thermal-hydraulics transient and the aerosol behavior of

Table 2 Spray characteristics

Activation time 111218 sDeactivation time 112118 sDroplet diameter 12119864 minus 4mMass flow rate 1417119864 minus 3m3s

Table 3 Thermal-hydraulic sensitivity cases performed

Case Characteristic Length value [m]1 0012 00153 0024 0035 0046 0057 0068 0079 00810 00911 01

the FPT-0 test This model consists of 6 CVs plus one tosimulate the outer environment 7 FLPs and 12 HSs

The CVs cut the main part of the cylindrical vessel intwo radial rings while the CV simulating the sump (SUMP)has a peculiar geometrical shape to avoid a too small gaseousphase when the water level increases to about 06m Atoo small sump gaseous phase could lead to a time-stepdecrease as well as abnormal calculation terminations dueto thermal-hydraulic instabilities (CVs only filled with watercannot be simulated in ASTEC [18]) To avoid this the CVsimulating the sump ldquoentersrdquo for about 04m into the abovevessel bottom zone to have a sufficient gaseous zone in anycircumstance The sump has an initial water inventory of100 kg and the gaseous atmosphere is filled with a mixtureof 525 of O

2and 9475 of N

2in MELCOR (as reported

in the FPT-0 final report) and with common air in ASTECThis small difference has a negligible influence on the overallresults obtained as stressed in the following FP sensitivityanalyses section The sump water PH has been set accordingto the data reported in the FPT-0 final report [9]

The 7 FLPs connect the different CVs while the 8HSs simulate the sump outer wall (WSU) the semi-ellipticbottom and top of the vessel (WB1 W2T and W4T) thecylindrical outer wall (W3 W4) and the three condensers(WET and DRY) The four remaining HSs were utilizedto provide an aerosol settling structure on the bottom ofthe C1 C2 C3 and C4 CVs as suggested in the MELCORUserrsquos Guide [21] Except WET and DRY all these HSs weresimulated with an imposed outer temperature evolving intime according to [9] with an inner temperature based on theheat exchange with their surrounding CVsThe characteristiclengths (CLs) were calculated as reported in (1) (the CL isexpressed as 119871 in (1)) The first correlation is taken from theCPA theory manual [18] and from a Sandia report on thenodalization of PWR containments [22] while the second is

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

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International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

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Nuclear InstallationsScience and Technology of

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Solar EnergyJournal of

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Wind EnergyJournal of

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Nuclear EnergyInternational Journal of

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High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 2: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

2 Science and Technology of Nuclear Installations

Model ofreactor core

Verticalline

Testassembly

core

Experimental cell

Horizontalline (hot leg)

700∘C 150∘C

Steam generatorU-tube

Horizontalline (cold leg)

(10G3)

Hot leg Cold leg

Model of primary circuitincluding steam generator

Sump

Paintedcoupon

Condensingsurfaces

Dry surfaces

Paintedcondensers

Paintedliners

Sump

Model of reactorcontainment

Fission Product Caisson

Ph bus containmentPh bus reactor

Ph bus reactor

Figure 1 Schematic overview of the Phebus facility (original image taken from [3])

nodalizations [10ndash15] not able to catch the main thermal-hydraulics or aerosol phenomena or to be employed forfull-plant analysis For this purpose three different spatialnodalizations of the Phebus containment vessel have beendeveloped to show that too simple nodalizations are notable to simulate the thermal-hydraulic transientThe simplestmodel consists in 7 control volumes (CVs) 12 heat structures(HSs) and 7 flow paths (FLPs) while the most complex oneconsists in 21 CVs 33 HSs and 32 FLPs These three nodal-izations have been kept as simple as possible (as the numberof CVs and HSs) to allow a similar spatial schematizationalso for full plant analysisThese nodalizations have been alsodeveloped in the most identical way possible for both codesbut the differentmodelling approaches of certain aspectswereall exploited to obtain the best possible results with eachcode The present paper is a part of a series of publicationscovering the 4 Phebus tests executed with a solid PWR fuelbundle FPT-0 FPT-1 FPT-2 and FPT-3 The aim is to writea detailed paper for each Phebus test to underline and discussthe main aspects of the containment behavior in the differenttestrsquos conditions For this reason these papers will follow asimilar strategy in the closest possible way An additionalpaper is also planned to summarize themain findings of thesefour works providing also a comparison on the influenceof each specific sensitivity parameters investigated on eachspecific test Of these five papers the one covering the FPT-1 test has been already published [16] Observations madeduring these analyses led to a thorough understanding of thein-containment source term and its coupling with thermal-hydraulics under unsaturated but condensing atmospheres

2 The Pheacutebus Facility

The Phebus facility is downscaled to a size of 5000 1 ofa typical French 900 MWe class pressurized water reactor(PWR) A schematic sketch of the facility is shown in Figure 1[3] The first component is the driver core and its coolingcircuit encapsulated inside a cylindrical shroud The coreconsists of a PWR fuel bundle two instrumented fuel rodsand a control rod Different fuel burnup levels and controlrod materials were investigated in each Phebus test Duringthese tests the driver core is heated up and irradiated torecreate the temperature increase supposed to occur duringa loss of coolant accident (LOCA) and the FPs build-upduring normal operations At the bottom of the shroud a lineis installed to inject steam Different steam injection slopeswere investigated in each test to analyze the fuel behaviorunder strongly or weakly oxidizing conditions At the topof the shroud the experimental line (EL) transports steamincondensable gases and FPs into the containmentThe EL issubdivided into three parts

(i) the first part made of Inconel-600 simulates a PWRhot leg

(ii) the second part still made of Inconel-600 simulatesa U-tube type SG

(iii) the third part made of AISI 304L simulates a PWRcold leg

Finally the EL enters into the center of the containment vesselto simulate a cold leg LOCA

Science and Technology of Nuclear Installations 3

The containment vessel has a total free volume of 10m3with a height of about 5m and an inner diameter of about18m The containment has a cylindrical shape with a watersump in the lower part The sump has a height of 06m andan inner diameter of 0584m to reproduce a representativeatmosphere water exchange surface [17] The top and thebottom parts of the vessel are closed with a semi-ellipsoidalstructure The containment walls are made of AISI 316L andtheir temperatures are controlled through two independentsystems respectively for the sump zone and for the ldquocylindri-calrdquo zone The aims of the two cooling loops are to decouplethe sumpwater temperature from the gas temperature and toavoid condensation onto containment walls A spray systemis also inserted in the lower containment zone to washdownthe FPs settled on the bottom vessel surfaces The sprayalmost covers the entire flow-through area of the cylindricalcontainment part and it is fed only by the water contained inthe sump

Three condensers are attached at the top vault to simulatethe cold structures of a reactor building The main aim ofthese condensers is to allow the control of the heat transferand steam condensation Each condenser has an externaldiameter of 015m a height of 25m and it is subdivided intotwo parts the ldquodryrdquo part of 0782m and the ldquowetrdquo part of1718m The total surface area of condensers is about 35m224m2 for the wet part and 11m2 for the dry part The innertemperatures of the two condenser halves are controlled bytwo independent loops to allow condensation only onto thewet part This wet part is also covered with an epoxy paintto allow the organic iodine formation Further details on thefacility can be found in the final report of the FPT-0 test [9]

3 The FPT-0 Test and Boundary Conditions

The Phebus FPT-0 test can be subdivided into four differentphases

(i) The degradation phase when the driver core is heatedup to allow the progressive melting of the fuel and thesubsequent release of FPs This phase starts at 0 s andends at 18138 s The fuel employed consists in a freshPWR rod bundle irradiated for a very short period toallow the formation of short-living FPs with a Ag-In-Cd control rod The degradation occurs in a stronglyoxidizing atmosphere

(ii) The aerosol phase with the containment maintainedin stable conditions and isolated from the driver core(at 20438 s) This phase starts at 20438 s and ends at86018 s

(iii) The washing phase with the activation of the sprayinside the containment to wash down the FPs settledonto the elliptic bottom The FPs are then collectedinside the sumpThis phase starts at 111218 s and endsat 112118 s

(iv) The chemistry phase During this last phase nocondensation occurs and the iodine chemistry in thesump is analyzed This phase starts at 133718 s andends at 433848 s that is till the termination of thetest

01

02

03

04

05

06

07

017

018

019

02

021

022

023

0 5000 10000 15000 20000 25000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 2 Total pressure and rh evolutions during the first 25000 sof the test

0

005

01

015

02

025

0

05

1

15

2

25

3

35

0 5000 10000 15000 20000 25000

Stea

m m

ass fl

ow ra

te (g

s)

Time (s)

Steam injection

(2

mas

s flow

rate

(gs

)

(2 injection

Figure 3 Steam and H2injection flow rates during the test

In Figure 2 the containment total pressure and the relativehumidity (rh) trends during the first 25000 s of the FPT-0 test are shown The data provided inside the FPT-0 finalreport [9] do not cover the entire test and for the rh scarcedata points are also reported During the first 25000 s thepressure and the rh follow the evolution of the steam flowrate (Figure 3) After the termination of the steam injec-tion the pressure (Figure 4) remains almost constant until40000 s when the wet condensersrsquo temperature is increasedto 38315 K This increase lasts until the beginning of thewashing phase preparation activities (88358 s) Such actionsconsist in an initial decrease of the vessel cylindrical walltemperature followed by a subsequent increase the decreaseof the wet condensers and the sump wall temperaturesand the isolation of the vessel elliptic bottom from theprimary organic loops controlling the vessel cylindrical walltemperature (Figure 5)The primary organic loops are closedcircuits in which an organic coolant flows These loopsare equipped with heaters and pumps to set the vesselrsquoswalls temperatures according to the test schedule The sprayactivation triggers an abrupt containment total pressure and

4 Science and Technology of Nuclear Installations

01

02

03

04

05

06

07

011

013

015

017

019

021

023

0 50000 100000 150000 200000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 4 Total pressure and rh evolutions up to 20000 s after thebeginning of the test

300

320

340

360

380

400

420

0 50000 100000 150000 200000

Wal

l tem

pera

ture

(K)

Time (s)

SumpBottomVessel

Dry condenserWet condenser

Figure 5 Wall temperature evolutions during the first 200000 s ofthe test

rh decrease After the termination of the washing phasethe elliptic bottom is reconnected to the primary organicloop and the wet condensers and the sumpwall temperaturesstart to increase again for the following chemistry phase Thewall temperatures were all employed as boundary conditionsNo experimental data for the total pressure and the wallrsquostemperatures were provided in [9] after 180000 s During thislast phase (chemistry phase) only the rh was measured forthree times at 223718 s at 310118 s and at 396518 s A rhvalue of 30 was reported for the first two measurementswhile a value of 31 was shown for the third one

Twomain injections occur in the containment during thetest the steam and the H

2injections The steam is injected at

the bottom of the degraded fuel element but only a fractionflows in the containment due to the steam reaction with thefuel clad H

2is produced by the Zircaloy oxidation reaction

but in the present paper such phenomenon is not simulatedso the H

2mass flow rate is imposed as a boundary condition

as well as the steam flow rate into the containment (Figure 3)During the test several samplings are also performed toanalyze the composition of the containment atmosphere

375376377378379380381382383384385

0 10000 20000 30000 40000

Atm

osph

eric

tem

pera

ture

(K)

Time (s)

402 m30 m

232m1351m

Figure 6 Atmospheric temperatures at different containmentheights during the fire first 40000 s of the test

365

370

375

380

385

390

395

400

405

0 50000 100000 150000 200000Time (s)

Atm

osph

eric

tem

pera

ture

(K)

232m

Figure 7 Atmospheric temperature evolution at 232m up to200000 s

Figure 6 shows the mean atmospheric temperature atdifferent containment heights until 40000 s while in Figure 7the atmospheric temperature at 232m only until 200000 sis reported because no data were provided for the otherheights In Figure 8 the sumpwater PH evolution for the first200000 s of the test is reported Again the data provided forthe PH evolution do not cover the entire test In Figure 9 thetotal condensation rate on the wet condensers until 35000 sis shown (no condensation occurs after that time) Finallythe atmospheric and the sump dose ratesrsquo evolutions upto 200000 s are reported in Figure 10 After 180000 s thedose rates in the atmosphere and the sump were measuredonly at 277338 s and values of 0245Gys (atmosphere) and0127Gys (sump)were reported Amore detailed descriptionof the test can be found in the FPT-0 final report [9]

4 Codes Employed

As previously said two codes have been employed for thepresent paper ASTEC v20 revision 3 patch 3 and MELCOR

Science and Technology of Nuclear Installations 5

4

42

44

46

48

5

52

54

0 50000 100000 150000 200000Time (s)

PH

Sum

p pH

(mdash)

Figure 8 Sump PH evolution for the first 200000 s of the test

0

05

1

15

2

25

3

0 10000 20000 30000

Con

dens

atio

n ra

te (g

s)

Time (s)

Condensation rate

Figure 9 Total condensation rate during the first 40000 s of thetest

V216840 Both codes are capable of simulating a SA from theinitiating event to the release of FPs outside the containmentThey are based on a lumped-parameter approach where thespatial domain is subdivided into CVs connected throughFLPs Inside each CV a nonequilibrium condition is appliedfor the liquid and gaseous zones The circulation among thedifferent CVs is provided by the FLPs connecting them Asimplified momentum balance equation is introduced foreach FLP but still capable of catching the main circulationphenomena inside the primary system or the containmentBoth codes include also additional packagesmodules to cou-ple thermal-hydraulics with aerosolFP calculations Signifi-cant differences exist regarding the aerosol and FP treatmentin terms of models numerical approach and coupling withthermal-hydraulics but such differences will not be discussedin the present paper

ASTEC is jointly developed by IRSN (France) and GRS(Germany) The code is subdivided into different modulesbut in the present work only the CPA IODE and SYSINTmodules were employed The CPA module deals with thecontainment thermal-hydraulics and aerosol behavior andit is subdivided into three submodules THY for thermal-hydraulic analysis AFP for FP behavior and FIPHOST for FP

0 50000 100000 150000 200000

Dos

e rat

e (G

ys)

Time (s)

Atmospheric doseWater dose

1E + 02

1E + 01

1E + 00

1E minus 01

1E minus 02

1E minus 03

Figure 10 Atmospheric andwater dose rates in containment duringthe test

transport in containment [18] The IODE module deals withthe Iodine and Ruthenium chemistry in containment [19]Finally the SYSINT module manages safety systems basedon user inputs or plant conditions [20] Specific informationon the different models implemented inside each ASTECmodule can be found in the code manuals

MELCOR is developed by Sandia National Laboratory(USA)The code is subdivided into different packages but inthe present work only the control function (CF) the controlvolume hydrodynamic (CVH) the executive (EXEC) theflow path (FL) the heat structure (HS) the material proper-ties (MP) the noncondensible gas (NCG) the radionuclide(RN) the containment spray (SPR) and the tabular function(TF) packages were employed The CF and TF packages dealwith the tabular (TF) and control (CF) functions needed forthe correct activation of the other modules The EXEC pack-age manages the execution parameters of the simulation astime step beginning and ending simulation time output filesand so on The CVH and FL packages model the thermal-hydraulic behavior of coolant and incondensable gases TheHS package models the structures attached to the differentCVs The MP and NCG packages activate the structuralmaterials (MP) and the incondensable gases (NCG) neededfor the correct execution of the simulationsThe SPR packagemanages the sprays in the containment building Finally theRN package models the behavior of aerosols and vapors inthe whole plant from the release from the damaged fuelto the deposition on containment surfaces This packageincludes also a specific model to simulate the Iodine behaviorin gaseous and liquid phases Specific information on thedifferent models implemented in eachMELCORmodule canbe found in the code manuals [21]

5 Employed Nodalizations andBoundary Conditions

Three spatial nodalizations have been developed to simulatethe containment behavior during the FPT-0 test In Figure 11a sketch of the three spatial models is shown and inTables 1 2 and 3 the geometrical features and the initial

6 Science and Technology of Nuclear Installations

Table 1 HSrsquos characteristic lengths

Wall Characteristic length [m]Model 1

WSU 0146WB1 0416W3 2299W4 1007W2T 0146W4T 0416WET 015DRY 015

Model 2WSU 0146WB1 0416W7 0759W8 0759W9 0782W10 0504W11 0504W6T 0146W12T 0416WET4 015WET5 015WET6 015DRY 015

Model 3WSU 0146WB1 0265WB2 0142W13 0759W14 0759W15 0782W16 0504W17 0504W6T 0146W12T 0265W18T 0142WET4 015WET5 015WET6 015DRY 015

boundary conditions employed for these three models arereported [9] Identical total pressures (195 kPa) and rh (49)were employed for the three different nodalizations Theatmospheric and the sump temperatures of each CV were setaccording to the data coming from the sensors installed atdifferent vessel heights and the wall temperatures were setaccording to the boundary conditions shown in Figure 5

The first scheme (M1) has been employed to show thatnodalizations with few nodes are not capable of simulatingthe thermal-hydraulics transient and the aerosol behavior of

Table 2 Spray characteristics

Activation time 111218 sDeactivation time 112118 sDroplet diameter 12119864 minus 4mMass flow rate 1417119864 minus 3m3s

Table 3 Thermal-hydraulic sensitivity cases performed

Case Characteristic Length value [m]1 0012 00153 0024 0035 0046 0057 0068 0079 00810 00911 01

the FPT-0 test This model consists of 6 CVs plus one tosimulate the outer environment 7 FLPs and 12 HSs

The CVs cut the main part of the cylindrical vessel intwo radial rings while the CV simulating the sump (SUMP)has a peculiar geometrical shape to avoid a too small gaseousphase when the water level increases to about 06m Atoo small sump gaseous phase could lead to a time-stepdecrease as well as abnormal calculation terminations dueto thermal-hydraulic instabilities (CVs only filled with watercannot be simulated in ASTEC [18]) To avoid this the CVsimulating the sump ldquoentersrdquo for about 04m into the abovevessel bottom zone to have a sufficient gaseous zone in anycircumstance The sump has an initial water inventory of100 kg and the gaseous atmosphere is filled with a mixtureof 525 of O

2and 9475 of N

2in MELCOR (as reported

in the FPT-0 final report) and with common air in ASTECThis small difference has a negligible influence on the overallresults obtained as stressed in the following FP sensitivityanalyses section The sump water PH has been set accordingto the data reported in the FPT-0 final report [9]

The 7 FLPs connect the different CVs while the 8HSs simulate the sump outer wall (WSU) the semi-ellipticbottom and top of the vessel (WB1 W2T and W4T) thecylindrical outer wall (W3 W4) and the three condensers(WET and DRY) The four remaining HSs were utilizedto provide an aerosol settling structure on the bottom ofthe C1 C2 C3 and C4 CVs as suggested in the MELCORUserrsquos Guide [21] Except WET and DRY all these HSs weresimulated with an imposed outer temperature evolving intime according to [9] with an inner temperature based on theheat exchange with their surrounding CVsThe characteristiclengths (CLs) were calculated as reported in (1) (the CL isexpressed as 119871 in (1)) The first correlation is taken from theCPA theory manual [18] and from a Sandia report on thenodalization of PWR containments [22] while the second is

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 3: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 3

The containment vessel has a total free volume of 10m3with a height of about 5m and an inner diameter of about18m The containment has a cylindrical shape with a watersump in the lower part The sump has a height of 06m andan inner diameter of 0584m to reproduce a representativeatmosphere water exchange surface [17] The top and thebottom parts of the vessel are closed with a semi-ellipsoidalstructure The containment walls are made of AISI 316L andtheir temperatures are controlled through two independentsystems respectively for the sump zone and for the ldquocylindri-calrdquo zone The aims of the two cooling loops are to decouplethe sumpwater temperature from the gas temperature and toavoid condensation onto containment walls A spray systemis also inserted in the lower containment zone to washdownthe FPs settled on the bottom vessel surfaces The sprayalmost covers the entire flow-through area of the cylindricalcontainment part and it is fed only by the water contained inthe sump

Three condensers are attached at the top vault to simulatethe cold structures of a reactor building The main aim ofthese condensers is to allow the control of the heat transferand steam condensation Each condenser has an externaldiameter of 015m a height of 25m and it is subdivided intotwo parts the ldquodryrdquo part of 0782m and the ldquowetrdquo part of1718m The total surface area of condensers is about 35m224m2 for the wet part and 11m2 for the dry part The innertemperatures of the two condenser halves are controlled bytwo independent loops to allow condensation only onto thewet part This wet part is also covered with an epoxy paintto allow the organic iodine formation Further details on thefacility can be found in the final report of the FPT-0 test [9]

3 The FPT-0 Test and Boundary Conditions

The Phebus FPT-0 test can be subdivided into four differentphases

(i) The degradation phase when the driver core is heatedup to allow the progressive melting of the fuel and thesubsequent release of FPs This phase starts at 0 s andends at 18138 s The fuel employed consists in a freshPWR rod bundle irradiated for a very short period toallow the formation of short-living FPs with a Ag-In-Cd control rod The degradation occurs in a stronglyoxidizing atmosphere

(ii) The aerosol phase with the containment maintainedin stable conditions and isolated from the driver core(at 20438 s) This phase starts at 20438 s and ends at86018 s

(iii) The washing phase with the activation of the sprayinside the containment to wash down the FPs settledonto the elliptic bottom The FPs are then collectedinside the sumpThis phase starts at 111218 s and endsat 112118 s

(iv) The chemistry phase During this last phase nocondensation occurs and the iodine chemistry in thesump is analyzed This phase starts at 133718 s andends at 433848 s that is till the termination of thetest

01

02

03

04

05

06

07

017

018

019

02

021

022

023

0 5000 10000 15000 20000 25000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 2 Total pressure and rh evolutions during the first 25000 sof the test

0

005

01

015

02

025

0

05

1

15

2

25

3

35

0 5000 10000 15000 20000 25000

Stea

m m

ass fl

ow ra

te (g

s)

Time (s)

Steam injection

(2

mas

s flow

rate

(gs

)

(2 injection

Figure 3 Steam and H2injection flow rates during the test

In Figure 2 the containment total pressure and the relativehumidity (rh) trends during the first 25000 s of the FPT-0 test are shown The data provided inside the FPT-0 finalreport [9] do not cover the entire test and for the rh scarcedata points are also reported During the first 25000 s thepressure and the rh follow the evolution of the steam flowrate (Figure 3) After the termination of the steam injec-tion the pressure (Figure 4) remains almost constant until40000 s when the wet condensersrsquo temperature is increasedto 38315 K This increase lasts until the beginning of thewashing phase preparation activities (88358 s) Such actionsconsist in an initial decrease of the vessel cylindrical walltemperature followed by a subsequent increase the decreaseof the wet condensers and the sump wall temperaturesand the isolation of the vessel elliptic bottom from theprimary organic loops controlling the vessel cylindrical walltemperature (Figure 5)The primary organic loops are closedcircuits in which an organic coolant flows These loopsare equipped with heaters and pumps to set the vesselrsquoswalls temperatures according to the test schedule The sprayactivation triggers an abrupt containment total pressure and

4 Science and Technology of Nuclear Installations

01

02

03

04

05

06

07

011

013

015

017

019

021

023

0 50000 100000 150000 200000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 4 Total pressure and rh evolutions up to 20000 s after thebeginning of the test

300

320

340

360

380

400

420

0 50000 100000 150000 200000

Wal

l tem

pera

ture

(K)

Time (s)

SumpBottomVessel

Dry condenserWet condenser

Figure 5 Wall temperature evolutions during the first 200000 s ofthe test

rh decrease After the termination of the washing phasethe elliptic bottom is reconnected to the primary organicloop and the wet condensers and the sumpwall temperaturesstart to increase again for the following chemistry phase Thewall temperatures were all employed as boundary conditionsNo experimental data for the total pressure and the wallrsquostemperatures were provided in [9] after 180000 s During thislast phase (chemistry phase) only the rh was measured forthree times at 223718 s at 310118 s and at 396518 s A rhvalue of 30 was reported for the first two measurementswhile a value of 31 was shown for the third one

Twomain injections occur in the containment during thetest the steam and the H

2injections The steam is injected at

the bottom of the degraded fuel element but only a fractionflows in the containment due to the steam reaction with thefuel clad H

2is produced by the Zircaloy oxidation reaction

but in the present paper such phenomenon is not simulatedso the H

2mass flow rate is imposed as a boundary condition

as well as the steam flow rate into the containment (Figure 3)During the test several samplings are also performed toanalyze the composition of the containment atmosphere

375376377378379380381382383384385

0 10000 20000 30000 40000

Atm

osph

eric

tem

pera

ture

(K)

Time (s)

402 m30 m

232m1351m

Figure 6 Atmospheric temperatures at different containmentheights during the fire first 40000 s of the test

365

370

375

380

385

390

395

400

405

0 50000 100000 150000 200000Time (s)

Atm

osph

eric

tem

pera

ture

(K)

232m

Figure 7 Atmospheric temperature evolution at 232m up to200000 s

Figure 6 shows the mean atmospheric temperature atdifferent containment heights until 40000 s while in Figure 7the atmospheric temperature at 232m only until 200000 sis reported because no data were provided for the otherheights In Figure 8 the sumpwater PH evolution for the first200000 s of the test is reported Again the data provided forthe PH evolution do not cover the entire test In Figure 9 thetotal condensation rate on the wet condensers until 35000 sis shown (no condensation occurs after that time) Finallythe atmospheric and the sump dose ratesrsquo evolutions upto 200000 s are reported in Figure 10 After 180000 s thedose rates in the atmosphere and the sump were measuredonly at 277338 s and values of 0245Gys (atmosphere) and0127Gys (sump)were reported Amore detailed descriptionof the test can be found in the FPT-0 final report [9]

4 Codes Employed

As previously said two codes have been employed for thepresent paper ASTEC v20 revision 3 patch 3 and MELCOR

Science and Technology of Nuclear Installations 5

4

42

44

46

48

5

52

54

0 50000 100000 150000 200000Time (s)

PH

Sum

p pH

(mdash)

Figure 8 Sump PH evolution for the first 200000 s of the test

0

05

1

15

2

25

3

0 10000 20000 30000

Con

dens

atio

n ra

te (g

s)

Time (s)

Condensation rate

Figure 9 Total condensation rate during the first 40000 s of thetest

V216840 Both codes are capable of simulating a SA from theinitiating event to the release of FPs outside the containmentThey are based on a lumped-parameter approach where thespatial domain is subdivided into CVs connected throughFLPs Inside each CV a nonequilibrium condition is appliedfor the liquid and gaseous zones The circulation among thedifferent CVs is provided by the FLPs connecting them Asimplified momentum balance equation is introduced foreach FLP but still capable of catching the main circulationphenomena inside the primary system or the containmentBoth codes include also additional packagesmodules to cou-ple thermal-hydraulics with aerosolFP calculations Signifi-cant differences exist regarding the aerosol and FP treatmentin terms of models numerical approach and coupling withthermal-hydraulics but such differences will not be discussedin the present paper

ASTEC is jointly developed by IRSN (France) and GRS(Germany) The code is subdivided into different modulesbut in the present work only the CPA IODE and SYSINTmodules were employed The CPA module deals with thecontainment thermal-hydraulics and aerosol behavior andit is subdivided into three submodules THY for thermal-hydraulic analysis AFP for FP behavior and FIPHOST for FP

0 50000 100000 150000 200000

Dos

e rat

e (G

ys)

Time (s)

Atmospheric doseWater dose

1E + 02

1E + 01

1E + 00

1E minus 01

1E minus 02

1E minus 03

Figure 10 Atmospheric andwater dose rates in containment duringthe test

transport in containment [18] The IODE module deals withthe Iodine and Ruthenium chemistry in containment [19]Finally the SYSINT module manages safety systems basedon user inputs or plant conditions [20] Specific informationon the different models implemented inside each ASTECmodule can be found in the code manuals

MELCOR is developed by Sandia National Laboratory(USA)The code is subdivided into different packages but inthe present work only the control function (CF) the controlvolume hydrodynamic (CVH) the executive (EXEC) theflow path (FL) the heat structure (HS) the material proper-ties (MP) the noncondensible gas (NCG) the radionuclide(RN) the containment spray (SPR) and the tabular function(TF) packages were employed The CF and TF packages dealwith the tabular (TF) and control (CF) functions needed forthe correct activation of the other modules The EXEC pack-age manages the execution parameters of the simulation astime step beginning and ending simulation time output filesand so on The CVH and FL packages model the thermal-hydraulic behavior of coolant and incondensable gases TheHS package models the structures attached to the differentCVs The MP and NCG packages activate the structuralmaterials (MP) and the incondensable gases (NCG) neededfor the correct execution of the simulationsThe SPR packagemanages the sprays in the containment building Finally theRN package models the behavior of aerosols and vapors inthe whole plant from the release from the damaged fuelto the deposition on containment surfaces This packageincludes also a specific model to simulate the Iodine behaviorin gaseous and liquid phases Specific information on thedifferent models implemented in eachMELCORmodule canbe found in the code manuals [21]

5 Employed Nodalizations andBoundary Conditions

Three spatial nodalizations have been developed to simulatethe containment behavior during the FPT-0 test In Figure 11a sketch of the three spatial models is shown and inTables 1 2 and 3 the geometrical features and the initial

6 Science and Technology of Nuclear Installations

Table 1 HSrsquos characteristic lengths

Wall Characteristic length [m]Model 1

WSU 0146WB1 0416W3 2299W4 1007W2T 0146W4T 0416WET 015DRY 015

Model 2WSU 0146WB1 0416W7 0759W8 0759W9 0782W10 0504W11 0504W6T 0146W12T 0416WET4 015WET5 015WET6 015DRY 015

Model 3WSU 0146WB1 0265WB2 0142W13 0759W14 0759W15 0782W16 0504W17 0504W6T 0146W12T 0265W18T 0142WET4 015WET5 015WET6 015DRY 015

boundary conditions employed for these three models arereported [9] Identical total pressures (195 kPa) and rh (49)were employed for the three different nodalizations Theatmospheric and the sump temperatures of each CV were setaccording to the data coming from the sensors installed atdifferent vessel heights and the wall temperatures were setaccording to the boundary conditions shown in Figure 5

The first scheme (M1) has been employed to show thatnodalizations with few nodes are not capable of simulatingthe thermal-hydraulics transient and the aerosol behavior of

Table 2 Spray characteristics

Activation time 111218 sDeactivation time 112118 sDroplet diameter 12119864 minus 4mMass flow rate 1417119864 minus 3m3s

Table 3 Thermal-hydraulic sensitivity cases performed

Case Characteristic Length value [m]1 0012 00153 0024 0035 0046 0057 0068 0079 00810 00911 01

the FPT-0 test This model consists of 6 CVs plus one tosimulate the outer environment 7 FLPs and 12 HSs

The CVs cut the main part of the cylindrical vessel intwo radial rings while the CV simulating the sump (SUMP)has a peculiar geometrical shape to avoid a too small gaseousphase when the water level increases to about 06m Atoo small sump gaseous phase could lead to a time-stepdecrease as well as abnormal calculation terminations dueto thermal-hydraulic instabilities (CVs only filled with watercannot be simulated in ASTEC [18]) To avoid this the CVsimulating the sump ldquoentersrdquo for about 04m into the abovevessel bottom zone to have a sufficient gaseous zone in anycircumstance The sump has an initial water inventory of100 kg and the gaseous atmosphere is filled with a mixtureof 525 of O

2and 9475 of N

2in MELCOR (as reported

in the FPT-0 final report) and with common air in ASTECThis small difference has a negligible influence on the overallresults obtained as stressed in the following FP sensitivityanalyses section The sump water PH has been set accordingto the data reported in the FPT-0 final report [9]

The 7 FLPs connect the different CVs while the 8HSs simulate the sump outer wall (WSU) the semi-ellipticbottom and top of the vessel (WB1 W2T and W4T) thecylindrical outer wall (W3 W4) and the three condensers(WET and DRY) The four remaining HSs were utilizedto provide an aerosol settling structure on the bottom ofthe C1 C2 C3 and C4 CVs as suggested in the MELCORUserrsquos Guide [21] Except WET and DRY all these HSs weresimulated with an imposed outer temperature evolving intime according to [9] with an inner temperature based on theheat exchange with their surrounding CVsThe characteristiclengths (CLs) were calculated as reported in (1) (the CL isexpressed as 119871 in (1)) The first correlation is taken from theCPA theory manual [18] and from a Sandia report on thenodalization of PWR containments [22] while the second is

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

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Journal ofPetroleum Engineering

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Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Journal of

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Renewable Energy

Submit your manuscripts athttpswwwhindawicom

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Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

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Nuclear InstallationsScience and Technology of

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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 4: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

4 Science and Technology of Nuclear Installations

01

02

03

04

05

06

07

011

013

015

017

019

021

023

0 50000 100000 150000 200000

Tota

l pre

ssur

e (M

Pa)

Time (s)

Total pressureRelative humidity

Relat

ive h

umid

ity (mdash

)

Figure 4 Total pressure and rh evolutions up to 20000 s after thebeginning of the test

300

320

340

360

380

400

420

0 50000 100000 150000 200000

Wal

l tem

pera

ture

(K)

Time (s)

SumpBottomVessel

Dry condenserWet condenser

Figure 5 Wall temperature evolutions during the first 200000 s ofthe test

rh decrease After the termination of the washing phasethe elliptic bottom is reconnected to the primary organicloop and the wet condensers and the sumpwall temperaturesstart to increase again for the following chemistry phase Thewall temperatures were all employed as boundary conditionsNo experimental data for the total pressure and the wallrsquostemperatures were provided in [9] after 180000 s During thislast phase (chemistry phase) only the rh was measured forthree times at 223718 s at 310118 s and at 396518 s A rhvalue of 30 was reported for the first two measurementswhile a value of 31 was shown for the third one

Twomain injections occur in the containment during thetest the steam and the H

2injections The steam is injected at

the bottom of the degraded fuel element but only a fractionflows in the containment due to the steam reaction with thefuel clad H

2is produced by the Zircaloy oxidation reaction

but in the present paper such phenomenon is not simulatedso the H

2mass flow rate is imposed as a boundary condition

as well as the steam flow rate into the containment (Figure 3)During the test several samplings are also performed toanalyze the composition of the containment atmosphere

375376377378379380381382383384385

0 10000 20000 30000 40000

Atm

osph

eric

tem

pera

ture

(K)

Time (s)

402 m30 m

232m1351m

Figure 6 Atmospheric temperatures at different containmentheights during the fire first 40000 s of the test

365

370

375

380

385

390

395

400

405

0 50000 100000 150000 200000Time (s)

Atm

osph

eric

tem

pera

ture

(K)

232m

Figure 7 Atmospheric temperature evolution at 232m up to200000 s

Figure 6 shows the mean atmospheric temperature atdifferent containment heights until 40000 s while in Figure 7the atmospheric temperature at 232m only until 200000 sis reported because no data were provided for the otherheights In Figure 8 the sumpwater PH evolution for the first200000 s of the test is reported Again the data provided forthe PH evolution do not cover the entire test In Figure 9 thetotal condensation rate on the wet condensers until 35000 sis shown (no condensation occurs after that time) Finallythe atmospheric and the sump dose ratesrsquo evolutions upto 200000 s are reported in Figure 10 After 180000 s thedose rates in the atmosphere and the sump were measuredonly at 277338 s and values of 0245Gys (atmosphere) and0127Gys (sump)were reported Amore detailed descriptionof the test can be found in the FPT-0 final report [9]

4 Codes Employed

As previously said two codes have been employed for thepresent paper ASTEC v20 revision 3 patch 3 and MELCOR

Science and Technology of Nuclear Installations 5

4

42

44

46

48

5

52

54

0 50000 100000 150000 200000Time (s)

PH

Sum

p pH

(mdash)

Figure 8 Sump PH evolution for the first 200000 s of the test

0

05

1

15

2

25

3

0 10000 20000 30000

Con

dens

atio

n ra

te (g

s)

Time (s)

Condensation rate

Figure 9 Total condensation rate during the first 40000 s of thetest

V216840 Both codes are capable of simulating a SA from theinitiating event to the release of FPs outside the containmentThey are based on a lumped-parameter approach where thespatial domain is subdivided into CVs connected throughFLPs Inside each CV a nonequilibrium condition is appliedfor the liquid and gaseous zones The circulation among thedifferent CVs is provided by the FLPs connecting them Asimplified momentum balance equation is introduced foreach FLP but still capable of catching the main circulationphenomena inside the primary system or the containmentBoth codes include also additional packagesmodules to cou-ple thermal-hydraulics with aerosolFP calculations Signifi-cant differences exist regarding the aerosol and FP treatmentin terms of models numerical approach and coupling withthermal-hydraulics but such differences will not be discussedin the present paper

ASTEC is jointly developed by IRSN (France) and GRS(Germany) The code is subdivided into different modulesbut in the present work only the CPA IODE and SYSINTmodules were employed The CPA module deals with thecontainment thermal-hydraulics and aerosol behavior andit is subdivided into three submodules THY for thermal-hydraulic analysis AFP for FP behavior and FIPHOST for FP

0 50000 100000 150000 200000

Dos

e rat

e (G

ys)

Time (s)

Atmospheric doseWater dose

1E + 02

1E + 01

1E + 00

1E minus 01

1E minus 02

1E minus 03

Figure 10 Atmospheric andwater dose rates in containment duringthe test

transport in containment [18] The IODE module deals withthe Iodine and Ruthenium chemistry in containment [19]Finally the SYSINT module manages safety systems basedon user inputs or plant conditions [20] Specific informationon the different models implemented inside each ASTECmodule can be found in the code manuals

MELCOR is developed by Sandia National Laboratory(USA)The code is subdivided into different packages but inthe present work only the control function (CF) the controlvolume hydrodynamic (CVH) the executive (EXEC) theflow path (FL) the heat structure (HS) the material proper-ties (MP) the noncondensible gas (NCG) the radionuclide(RN) the containment spray (SPR) and the tabular function(TF) packages were employed The CF and TF packages dealwith the tabular (TF) and control (CF) functions needed forthe correct activation of the other modules The EXEC pack-age manages the execution parameters of the simulation astime step beginning and ending simulation time output filesand so on The CVH and FL packages model the thermal-hydraulic behavior of coolant and incondensable gases TheHS package models the structures attached to the differentCVs The MP and NCG packages activate the structuralmaterials (MP) and the incondensable gases (NCG) neededfor the correct execution of the simulationsThe SPR packagemanages the sprays in the containment building Finally theRN package models the behavior of aerosols and vapors inthe whole plant from the release from the damaged fuelto the deposition on containment surfaces This packageincludes also a specific model to simulate the Iodine behaviorin gaseous and liquid phases Specific information on thedifferent models implemented in eachMELCORmodule canbe found in the code manuals [21]

5 Employed Nodalizations andBoundary Conditions

Three spatial nodalizations have been developed to simulatethe containment behavior during the FPT-0 test In Figure 11a sketch of the three spatial models is shown and inTables 1 2 and 3 the geometrical features and the initial

6 Science and Technology of Nuclear Installations

Table 1 HSrsquos characteristic lengths

Wall Characteristic length [m]Model 1

WSU 0146WB1 0416W3 2299W4 1007W2T 0146W4T 0416WET 015DRY 015

Model 2WSU 0146WB1 0416W7 0759W8 0759W9 0782W10 0504W11 0504W6T 0146W12T 0416WET4 015WET5 015WET6 015DRY 015

Model 3WSU 0146WB1 0265WB2 0142W13 0759W14 0759W15 0782W16 0504W17 0504W6T 0146W12T 0265W18T 0142WET4 015WET5 015WET6 015DRY 015

boundary conditions employed for these three models arereported [9] Identical total pressures (195 kPa) and rh (49)were employed for the three different nodalizations Theatmospheric and the sump temperatures of each CV were setaccording to the data coming from the sensors installed atdifferent vessel heights and the wall temperatures were setaccording to the boundary conditions shown in Figure 5

The first scheme (M1) has been employed to show thatnodalizations with few nodes are not capable of simulatingthe thermal-hydraulics transient and the aerosol behavior of

Table 2 Spray characteristics

Activation time 111218 sDeactivation time 112118 sDroplet diameter 12119864 minus 4mMass flow rate 1417119864 minus 3m3s

Table 3 Thermal-hydraulic sensitivity cases performed

Case Characteristic Length value [m]1 0012 00153 0024 0035 0046 0057 0068 0079 00810 00911 01

the FPT-0 test This model consists of 6 CVs plus one tosimulate the outer environment 7 FLPs and 12 HSs

The CVs cut the main part of the cylindrical vessel intwo radial rings while the CV simulating the sump (SUMP)has a peculiar geometrical shape to avoid a too small gaseousphase when the water level increases to about 06m Atoo small sump gaseous phase could lead to a time-stepdecrease as well as abnormal calculation terminations dueto thermal-hydraulic instabilities (CVs only filled with watercannot be simulated in ASTEC [18]) To avoid this the CVsimulating the sump ldquoentersrdquo for about 04m into the abovevessel bottom zone to have a sufficient gaseous zone in anycircumstance The sump has an initial water inventory of100 kg and the gaseous atmosphere is filled with a mixtureof 525 of O

2and 9475 of N

2in MELCOR (as reported

in the FPT-0 final report) and with common air in ASTECThis small difference has a negligible influence on the overallresults obtained as stressed in the following FP sensitivityanalyses section The sump water PH has been set accordingto the data reported in the FPT-0 final report [9]

The 7 FLPs connect the different CVs while the 8HSs simulate the sump outer wall (WSU) the semi-ellipticbottom and top of the vessel (WB1 W2T and W4T) thecylindrical outer wall (W3 W4) and the three condensers(WET and DRY) The four remaining HSs were utilizedto provide an aerosol settling structure on the bottom ofthe C1 C2 C3 and C4 CVs as suggested in the MELCORUserrsquos Guide [21] Except WET and DRY all these HSs weresimulated with an imposed outer temperature evolving intime according to [9] with an inner temperature based on theheat exchange with their surrounding CVsThe characteristiclengths (CLs) were calculated as reported in (1) (the CL isexpressed as 119871 in (1)) The first correlation is taken from theCPA theory manual [18] and from a Sandia report on thenodalization of PWR containments [22] while the second is

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 5: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 5

4

42

44

46

48

5

52

54

0 50000 100000 150000 200000Time (s)

PH

Sum

p pH

(mdash)

Figure 8 Sump PH evolution for the first 200000 s of the test

0

05

1

15

2

25

3

0 10000 20000 30000

Con

dens

atio

n ra

te (g

s)

Time (s)

Condensation rate

Figure 9 Total condensation rate during the first 40000 s of thetest

V216840 Both codes are capable of simulating a SA from theinitiating event to the release of FPs outside the containmentThey are based on a lumped-parameter approach where thespatial domain is subdivided into CVs connected throughFLPs Inside each CV a nonequilibrium condition is appliedfor the liquid and gaseous zones The circulation among thedifferent CVs is provided by the FLPs connecting them Asimplified momentum balance equation is introduced foreach FLP but still capable of catching the main circulationphenomena inside the primary system or the containmentBoth codes include also additional packagesmodules to cou-ple thermal-hydraulics with aerosolFP calculations Signifi-cant differences exist regarding the aerosol and FP treatmentin terms of models numerical approach and coupling withthermal-hydraulics but such differences will not be discussedin the present paper

ASTEC is jointly developed by IRSN (France) and GRS(Germany) The code is subdivided into different modulesbut in the present work only the CPA IODE and SYSINTmodules were employed The CPA module deals with thecontainment thermal-hydraulics and aerosol behavior andit is subdivided into three submodules THY for thermal-hydraulic analysis AFP for FP behavior and FIPHOST for FP

0 50000 100000 150000 200000

Dos

e rat

e (G

ys)

Time (s)

Atmospheric doseWater dose

1E + 02

1E + 01

1E + 00

1E minus 01

1E minus 02

1E minus 03

Figure 10 Atmospheric andwater dose rates in containment duringthe test

transport in containment [18] The IODE module deals withthe Iodine and Ruthenium chemistry in containment [19]Finally the SYSINT module manages safety systems basedon user inputs or plant conditions [20] Specific informationon the different models implemented inside each ASTECmodule can be found in the code manuals

MELCOR is developed by Sandia National Laboratory(USA)The code is subdivided into different packages but inthe present work only the control function (CF) the controlvolume hydrodynamic (CVH) the executive (EXEC) theflow path (FL) the heat structure (HS) the material proper-ties (MP) the noncondensible gas (NCG) the radionuclide(RN) the containment spray (SPR) and the tabular function(TF) packages were employed The CF and TF packages dealwith the tabular (TF) and control (CF) functions needed forthe correct activation of the other modules The EXEC pack-age manages the execution parameters of the simulation astime step beginning and ending simulation time output filesand so on The CVH and FL packages model the thermal-hydraulic behavior of coolant and incondensable gases TheHS package models the structures attached to the differentCVs The MP and NCG packages activate the structuralmaterials (MP) and the incondensable gases (NCG) neededfor the correct execution of the simulationsThe SPR packagemanages the sprays in the containment building Finally theRN package models the behavior of aerosols and vapors inthe whole plant from the release from the damaged fuelto the deposition on containment surfaces This packageincludes also a specific model to simulate the Iodine behaviorin gaseous and liquid phases Specific information on thedifferent models implemented in eachMELCORmodule canbe found in the code manuals [21]

5 Employed Nodalizations andBoundary Conditions

Three spatial nodalizations have been developed to simulatethe containment behavior during the FPT-0 test In Figure 11a sketch of the three spatial models is shown and inTables 1 2 and 3 the geometrical features and the initial

6 Science and Technology of Nuclear Installations

Table 1 HSrsquos characteristic lengths

Wall Characteristic length [m]Model 1

WSU 0146WB1 0416W3 2299W4 1007W2T 0146W4T 0416WET 015DRY 015

Model 2WSU 0146WB1 0416W7 0759W8 0759W9 0782W10 0504W11 0504W6T 0146W12T 0416WET4 015WET5 015WET6 015DRY 015

Model 3WSU 0146WB1 0265WB2 0142W13 0759W14 0759W15 0782W16 0504W17 0504W6T 0146W12T 0265W18T 0142WET4 015WET5 015WET6 015DRY 015

boundary conditions employed for these three models arereported [9] Identical total pressures (195 kPa) and rh (49)were employed for the three different nodalizations Theatmospheric and the sump temperatures of each CV were setaccording to the data coming from the sensors installed atdifferent vessel heights and the wall temperatures were setaccording to the boundary conditions shown in Figure 5

The first scheme (M1) has been employed to show thatnodalizations with few nodes are not capable of simulatingthe thermal-hydraulics transient and the aerosol behavior of

Table 2 Spray characteristics

Activation time 111218 sDeactivation time 112118 sDroplet diameter 12119864 minus 4mMass flow rate 1417119864 minus 3m3s

Table 3 Thermal-hydraulic sensitivity cases performed

Case Characteristic Length value [m]1 0012 00153 0024 0035 0046 0057 0068 0079 00810 00911 01

the FPT-0 test This model consists of 6 CVs plus one tosimulate the outer environment 7 FLPs and 12 HSs

The CVs cut the main part of the cylindrical vessel intwo radial rings while the CV simulating the sump (SUMP)has a peculiar geometrical shape to avoid a too small gaseousphase when the water level increases to about 06m Atoo small sump gaseous phase could lead to a time-stepdecrease as well as abnormal calculation terminations dueto thermal-hydraulic instabilities (CVs only filled with watercannot be simulated in ASTEC [18]) To avoid this the CVsimulating the sump ldquoentersrdquo for about 04m into the abovevessel bottom zone to have a sufficient gaseous zone in anycircumstance The sump has an initial water inventory of100 kg and the gaseous atmosphere is filled with a mixtureof 525 of O

2and 9475 of N

2in MELCOR (as reported

in the FPT-0 final report) and with common air in ASTECThis small difference has a negligible influence on the overallresults obtained as stressed in the following FP sensitivityanalyses section The sump water PH has been set accordingto the data reported in the FPT-0 final report [9]

The 7 FLPs connect the different CVs while the 8HSs simulate the sump outer wall (WSU) the semi-ellipticbottom and top of the vessel (WB1 W2T and W4T) thecylindrical outer wall (W3 W4) and the three condensers(WET and DRY) The four remaining HSs were utilizedto provide an aerosol settling structure on the bottom ofthe C1 C2 C3 and C4 CVs as suggested in the MELCORUserrsquos Guide [21] Except WET and DRY all these HSs weresimulated with an imposed outer temperature evolving intime according to [9] with an inner temperature based on theheat exchange with their surrounding CVsThe characteristiclengths (CLs) were calculated as reported in (1) (the CL isexpressed as 119871 in (1)) The first correlation is taken from theCPA theory manual [18] and from a Sandia report on thenodalization of PWR containments [22] while the second is

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

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Nuclear InstallationsScience and Technology of

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Solar EnergyJournal of

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Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 6: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

6 Science and Technology of Nuclear Installations

Table 1 HSrsquos characteristic lengths

Wall Characteristic length [m]Model 1

WSU 0146WB1 0416W3 2299W4 1007W2T 0146W4T 0416WET 015DRY 015

Model 2WSU 0146WB1 0416W7 0759W8 0759W9 0782W10 0504W11 0504W6T 0146W12T 0416WET4 015WET5 015WET6 015DRY 015

Model 3WSU 0146WB1 0265WB2 0142W13 0759W14 0759W15 0782W16 0504W17 0504W6T 0146W12T 0265W18T 0142WET4 015WET5 015WET6 015DRY 015

boundary conditions employed for these three models arereported [9] Identical total pressures (195 kPa) and rh (49)were employed for the three different nodalizations Theatmospheric and the sump temperatures of each CV were setaccording to the data coming from the sensors installed atdifferent vessel heights and the wall temperatures were setaccording to the boundary conditions shown in Figure 5

The first scheme (M1) has been employed to show thatnodalizations with few nodes are not capable of simulatingthe thermal-hydraulics transient and the aerosol behavior of

Table 2 Spray characteristics

Activation time 111218 sDeactivation time 112118 sDroplet diameter 12119864 minus 4mMass flow rate 1417119864 minus 3m3s

Table 3 Thermal-hydraulic sensitivity cases performed

Case Characteristic Length value [m]1 0012 00153 0024 0035 0046 0057 0068 0079 00810 00911 01

the FPT-0 test This model consists of 6 CVs plus one tosimulate the outer environment 7 FLPs and 12 HSs

The CVs cut the main part of the cylindrical vessel intwo radial rings while the CV simulating the sump (SUMP)has a peculiar geometrical shape to avoid a too small gaseousphase when the water level increases to about 06m Atoo small sump gaseous phase could lead to a time-stepdecrease as well as abnormal calculation terminations dueto thermal-hydraulic instabilities (CVs only filled with watercannot be simulated in ASTEC [18]) To avoid this the CVsimulating the sump ldquoentersrdquo for about 04m into the abovevessel bottom zone to have a sufficient gaseous zone in anycircumstance The sump has an initial water inventory of100 kg and the gaseous atmosphere is filled with a mixtureof 525 of O

2and 9475 of N

2in MELCOR (as reported

in the FPT-0 final report) and with common air in ASTECThis small difference has a negligible influence on the overallresults obtained as stressed in the following FP sensitivityanalyses section The sump water PH has been set accordingto the data reported in the FPT-0 final report [9]

The 7 FLPs connect the different CVs while the 8HSs simulate the sump outer wall (WSU) the semi-ellipticbottom and top of the vessel (WB1 W2T and W4T) thecylindrical outer wall (W3 W4) and the three condensers(WET and DRY) The four remaining HSs were utilizedto provide an aerosol settling structure on the bottom ofthe C1 C2 C3 and C4 CVs as suggested in the MELCORUserrsquos Guide [21] Except WET and DRY all these HSs weresimulated with an imposed outer temperature evolving intime according to [9] with an inner temperature based on theheat exchange with their surrounding CVsThe characteristiclengths (CLs) were calculated as reported in (1) (the CL isexpressed as 119871 in (1)) The first correlation is taken from theCPA theory manual [18] and from a Sandia report on thenodalization of PWR containments [22] while the second is

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

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Solar EnergyJournal of

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Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 7: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 7

WET

SUMP

C1 C3

C4C2

00

06

1033

3332

505

W3

DRY

DRY

DRY

W4

WSU

W4TW2T

Water zone

WB1

43

B1

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W7

WSU

W12TW6T

Water zone

WB1

434

B1

17915

255

C2 C8

C3 C9

W8

W9

C11C5

C12C6

W10

W11

3816

WET

6W

ET5

WET

4

SUMP

C1 C7

C10C4

00

06

1033

3332

505

W13

WSU

W12TW6T

Water zone

WB1

4339

B2

17915

255

C2 C8

C3 C9

W14

W15

C11C5

C12C6

W16

W17

38355

B1

C13

C16

C14

C15

C17

C18

WB2

W18T

Model 1 (M1) Model 2 (M2) Model 3 (M3)0584 0584 0584

0584

1560

05840584

Figure 11 Sketch of the nodalizations employed

taken only from [22] because no information was given in[18]

Characteristic Length Relations

Vertical cylinders 997888rarr 119871 = Height of the wall

Horizontal surfaces 997888rarr 119871 = Area of the wallPerimeter of the wall

(1)

On the opposite the WET and DRY HSs were simulatedwith an inner temperature evolving in time according to [9]and with an outer temperature based on the conditions oftheir surroundingCVsThe characteristic lengths ofDRY andWET HSs were set equal to their external diameter (015m)because such dimension was found capable of predicting acondensation rate closer to the experimental one in all thethree models employed Such discrepancy is due to the filmcondensation model implemented in both codes which isnot suitable to describe the drop-wise condensation probablyoccurring on the condenser surfaces In MELCOR the issueof CL for a vertically oriented plate type HS was carefullydescribed in [23] The authors suggest setting the CL valueequal to the physical length of the plate but at the same timethey underline that this solution cannot be extended a priorito other HSrsquos geometries Even if a detailed investigation onthe ldquoCL valuerdquo issue similar to that performed in [23] wasnot performed in the present work the use of the outerdiameter value for a cylindrical wall with condensation on theouter surface can be suggested for MELCOR For the ASTECcode similar problems and explanations were also found in

[10] where CL values of about 001ndash002m were suggestedalthough in the present analysis the values of the sameorder of magnitude provided too strong condensation ratesOn the contrary a value of about 015m had shown goodcondensation rates (see thermal-hydraulics results section)Therefore the use of a CL value equal to the outer cylinderdiameter can be suggested for both codes This value has alsothe advantage to be a ldquorealrdquo dimension of the condensersbeing the outer condenser diameter Table 1 presents asummary of the CL values employed

The solutions proposed in the previous cited work andin this paper stress that the characteristic length of suchcondensers is still an open question and so a possible sourceof userrsquos effect on the calculation Further studies will benecessary to be able to draw general guidelines about thecharacteristic lengths of inner walls in a scaled facility

Additional FLPs andHSs were also introduced in ASTECandMELCOR to support the thermal-hydraulics and aerosolcalculations In both codes additional HSs on the bottom ofClowast CVs are added to provide a settling surface as suggestedin [21] Such HSs have no influence on the MELCORcalculations but in ASTEC they improve the overall aerosolresults In ASTEC three additional FLPs are also added oneto simulate the water draining from the CV containing thewet condensers into the sump and the other two to simulatethe spraysrsquo injection In MELCOR two additional HSs areadded to simulate the draining into the sump of the watercondensed on the wet condenser surfaces This approach isdue to the different draining models implemented in the twocodes ASTEC implements a simple draining model allowing

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

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Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Renewable Energy

Submit your manuscripts athttpswwwhindawicom

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StructuresJournal of

International Journal of

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

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Journal of

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Page 8: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

8 Science and Technology of Nuclear Installations

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

023

0 10000 20000 30000

Tota

l pre

ssur

e (M

Pa)

Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 12 Total pressure evolution during the degradation and the early aerosol phases

the discharge of water accumulated on the bottom of a CV toanother CV On the contrary MELCOR implements only amodel to track the water flow along vertical HSs (the so calledldquofilm tracking modelrdquo) until the ingress in the sump water Inthe present work this last model is applied to simulate theldquodirectrdquo transport of the condensed water from the bottom ofthe condenser HS (WET) into the bottom sump HS (WSU)However this model has not to be employed to simulate thedirect transport of water from an upper zone to a lowestzone but only the water flowing along vertical HSs althoughleaving the condensed water on the bottom of the C3 CVwould have led to poor thermal-hydraulics results

In MELCOR the spray injections are simulated witha specific package instead of atmospheric junctions as inASTEC The main spray characteristics are the same for thetwo codes (Table 2) and a filter has been added in both codesto block aerosols in the sump water

The steam H2 and FP injections as well as the different

samplings were placed in the C1 CV (in all the threemodels)and their slopes were set according to the data reported inthe FPT-0 final report [9] The whole FP injections weresimulated with an aerosol mass media diameter (AMMD) of13119864minus 6m and a geometric standard deviation (GSD) of 215The dose rate evolutions in atmosphere and in the sump havebeen imposed basing on the data reported in the FPT-0 finalreport [9]

The second spatial scheme (M2) is a more refined oneand it is partially able to reproduce the different phenomenaoccurring during the FPT-0 test The model consists of 14CVs plus one for the outer environment 19 FLPs and 25HSs The spatial subdivision of the vessel is more refinedbut the vessel cylindrical zone is still subdivided only intotwo radial rings As for the previous M1 scheme additionalHSs and FPs have been introduced in the model to simulate

the spray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations InASTEC five FLPs are schematized three FLPsto simulate the draining of water from the bottom of the CVcontaining the wet condenser surfaces into the sump andthe other two to simulate the spray injection In MELCORthe water draining from the upper HS (WET6) to the lowerHSs (WET5 andWET4) is correctly simulated but the directtransport of water from the upper zone (C4) into the sump isstill simulated as in M1 All other assumptions made for M1are also valid for M2

The third scheme (M3) is the most complex and it iscapable of correctly catching the main phenomena occurringin the containment vessel during the FPT-0 test In thismodel the vessel cylindrical zone is subdivided into threeradial rings and it consists in 21 CVs plus one for theouter environment 32 FLPs and 33 HSs As for M1 and M2schemes additional FLPs and HSs are added to simulate thespray injection the draining of the condensed water ontowet condenser surfaces and to support the aerosols and FPscalculations The remarks made for M1 and M2 are also validfor this model

6 Thermal-Hydraulics Results

The correctness of the thermal-hydraulics predictions is ofmajor importance because of its influence on the overallaerosols and FPs behavior especially on the iodine chemistry

The total pressure evolution inside the containment vesselis shown in Figures 12 13 and 14 During the degradationphase (Figure 12) themaximumdiscrepancy shown is alwaysbelow 0005MPa for both codes but major discrepanciesstart to appear in the following phases The increase of thewet condensers temperature from 73∘C to 100∘C is the main

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

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Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 9: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 9

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

017

018

019

020

021

022

35000 55000 75000 95000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 13 Total pressure evolution during the aerosol phase

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - AST

M2 - ASTM3 - AST

013

015

017

019

021

023

100000 200000 300000 400000

Tota

l pre

ssur

e (M

Pa)

Time (s)EXPM1 - MEL

M2 - MELM3 - MEL

Figure 14 Total pressure evolution during the washing and the chemistry phases

event occurring between 35000 s and 90000 s (Figure 5)This action leads to a total pressure increase from about0177MPa to about 0195MPa (Figure 13) but the two codesfail to correctly catch this increase overestimating the totalpressure at 90000 s of about 0015MPa Minor differencesexist between the predictions of the two codes hence thespatial nodalization employed may be the cause behind thisdiscrepancy From 90000 s to 100000 s different actions wereperformed the wall temperatures were increased from 100∘C

to 130∘C the wet condensersrsquo temperature was decreasedto 40∘C and the containment elliption bottom was isolatedfrom the loop controlling its temperature During this timeinterval both codes again fail to catch the experimental totalpressure trend probably because of an improper estima-tion of the influence of these different actions During theexperiment the vessel temperature increase seems capable ofmaintaining an almost constant pressure in the containmentvessel even if the wet condensers temperature starts to

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 10: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

10 Science and Technology of Nuclear Installations

98

100

102

104

106

108

110

0 10000 20000 30000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

EXPM1 - AST

M2 - ASTM3 - AST

Time (s)

98

100

102

104

106

108

110

0 10000 20000 30000

EXPM1 - MEL

M2 - MELM3 - MEL

Figure 15 Atmospheric temperature evolution at 232m during the degradation phase

decrease On the contrary in both codes the vessel walltemperature increase seems insufficient to maintain a stablecondition inside the containment vessel and hence the totalpressure starts to decrease This difference is due to thecombined effects of the performed nodalization choices andof the codersquos capabilities Since similar discrepancies areshown by both codes a common cause should lie behindthem that is the employed nodalization At the same timepreliminary analyses with newer codersquos versions showedimportant differences between the ASTEC and theMELCORcodes [24] Moreover imprecise information was reported inthe FPT-0 final report [9] in this timewindowDuring the iso-lation period (from 90938 s to 112418 s) no temperature datawere provided for the bottom wall temperature evolutionthus a constant temperature (100∘C) was then assumed Thisassumption surely introduces a nonnegligible user effect onthe calculations but the absence of experimental data forcesthis strategy

During the washing phase up to 140000 s the calculatedtotal pressure trends agree with the experimental one buta maximum error of about 001MPa still exists (Figure 14)In the following late chemistry phase (after 140000 s) somediscrepancies start to appear again leading to a total pressureoverestimation of about 001MPa in ASTEC and 002MPa inMELCORNo experimental datawere provided into the FPT-0 Final Report after 175000 s [9] Similar discrepancies areshown during the aerosol and the chemistry phases probablybecause similar actions are performed in these two phases(increase in the wet condenser temperature)

The atmospheric temperature evolutions during thedegradation phase at 232m 30m and 402m are shownin Figures 15 16 and 17 respectively At 232m and 30min ASTEC a maximum error of about 2∘C is shown whileat 402m the error increases to about 4∘C Such values are

almost comparable with the ones from other precedent worksby different authors [12 13] On the contrary in MELCORat 232m and 30m a maximum error of 35ndash4∘C is shownwhile at 402m the error decreases to about 15∘C

During the following phases the temperature experimen-tal data were provided only at 232m (Figure 18) ASTECshows quite good results except from 111000 s to 115000 swhen the temperature decreases to about 90∘C During theother phases the maximum error does not exceed 25∘COn the contrary MELCOR predicts quite poor results espe-cially around 100000 s when the atmospheric temperature isunderestimated of about 10∘CThe poor results shown by theMELCOR code had pushed to a thorough sensitivity analysispresented in the following thermal-hydraulics sensitivityanalyses section

In Figure 19 the relative humidity (rh) evolution duringthe degradation phase is reported Few experimental datapoints were provided but sufficient to highlight the evolutionof the rh during the variation of the steam injection ForMELCOR the atmospheric temperatures are mainly under-estimated so a rh slightly above the experimental values isreported although the values obtained are quite consistentwith the experimental ones especially employing theM2 andM3 models For ASTEC the atmospheric temperatures aremainly underestimated for the M1 model and overestimatedfor the remaining two models The rh predictions seem notto be influenced by these different predictions and similarrh values are shown during the first 35000 s of the test Thisresult is probably due to a codersquos effect since no physicalreasons can be found to explain it Amaximum error of about10 is shown at 20500 s This discrepancy is quite high andcannot be considered fully acceptable but it should also benoticed that it is shown for less than 2000 s

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 11: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 11

EXPM1 - AST

M2 - ASTM3 - AST

98

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

10000 20000 300000Time (s)

98

100

102

104

106

108

110At

mos

pher

ic te

mpe

ratu

re (∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)Figure 16 Atmospheric temperature evolution at 30m during the degradation phase

EXPM1 - AST

M2 - ASTM3 - AST

100

102

104

106

108

110

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

100

102

104

106

108

110

10000 20000 300000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 17 Atmospheric temperature evolution at 402m during the degradation phase

During the washing phase only two experimental datapoints for the rh were provided (Figure 20) In this case thelack of experimental data does not allow a careful estimationof the results obtained with both codes On the contraryduring the chemistry phases four experimental data pointswere provided ASTEC shows quite good results thanksto the good atmospheric temperature prediction shown inFigure 18 while MELCOR overestimates the rh of about 7-8 because the atmospheric temperature was not perfectly

matched during this period (Figure 18) Finally in Figure 21the condensation rate onto the wet condensers is reportedNo appreciable difference can be highlighted among theexperimental values and the code predictions

Anyway very few differences characterized the resultsobtained with the three models employing both codes Con-sidering only the thermal-hydraulic point of view the simplermodel (M1) can be considered the best choice thanks to thelower computation time needed to perform a calculation

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 12: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

12 Science and Technology of Nuclear Installations

EXPM1 - AST

M2 - ASTM3 - AST

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

90

95

100

105

110

115

120

125

130

85000 13500035000Time (s)

Atm

osph

eric

tem

pera

ture

(∘ C)

Atm

osph

eric

tem

pera

ture

(∘ C)

Figure 18 Atmospheric temperature evolution at 232m until 180000 s

EXPM1 - AST

M2 - ASTM3 - AST

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

030

040

050

060

070

080

Relat

ive h

umid

ity (mdash

)

10000 20000 300000Time (s)

Figure 19 Relative humidity evolution during the degradation phase

However similar thermal-hydraulics results did not meanthat the three models work in the same way and that theinternal atmosphere circulation is correctly reproduced

7 Thermal-Hydraulics Sensitivity Analyses

The poor atmospheric temperatures predicted by MELCORduring the washing phase had led to a thorough sensitivityanalysis to improve the test predictions On the contrarysuch analysis was not carried out for ASTEC because a

good agreement with the experimental data was obtainedDifferent input parameters have been investigated but onlythe characteristic lengths of the outer vessel surfaces werefound to be themain influencing parameter For this purposea careful analysis was carried out to find the value of thecharacteristic length providing the best-estimation of thethermal-hydraulic transient This analysis has been carriedout substituting the characteristic length values calculated forthe different outer wall structures (the characteristic lengthsfor the wet and dry condenser structures were not modified)

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 13: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 13

EXPM1 - AST

M2 - ASTM3 - AST

005

015

025

035

045

055

065

075Re

lativ

e hum

idity

(mdash)

135000 235000 33500035000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

135000 235000 33500035000Time (s)

Figure 20 Relative humidity evolution during the aerosol washing and chemistry phases

EXPM1 - AST

M2 - ASTM3 - AST

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

EXPM1 - MEL

M2 - MELM3 - MEL

0

05

1

15

2

25

3

Con

dens

atio

n ra

te (g

s)

10000 20000 300000Time (s)

Figure 21 Condensation rate on wet condensers during the degradation phase

with values spanning from 001m to 01m A common valuewas set for each outer vessel structure and a summary of thecases executed is shown in Table 3

The three spatial nodalizations have shown differentvalues for the best-estimate characteristic length 0015mprovided exhaustive results in M1 001m in M2 and 004min M3 A clear indication of the cause behind this behaviorcannot be drawn with only three spatial nodalizations Themain difference between M1 and M2 is the improved verticalsubdivision of the containment vessel but the effects of thisenhancement seem almost negligible for the characteristic

length best-estimation value On the contrary the radialsubdivision of the vessel seems more influencing For thispurpose an additional spatial model (M4) was createdsubdividing the containment vessel into three radial rings(the B2 C13 C14 C15 C16 C17 and C18 control volumesof M3 were each split into two independent CVs) With thisnew nodalization the best estimation characteristic lengthwas found to be 005mThe obtained improvement is almostsimilar to that obtained between M1 and M2 so probably acombined effect of increased vertical layers and radial ringssubdivision is necessary to increase the characteristic length

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

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Page 14: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

14 Science and Technology of Nuclear Installations

EXPM3 defaultM3 BE

013014015016017018019020021022023

Tota

l pre

ssur

e (M

Pa)

50000 100000 150000 2000000Time (s)

Figure 22 Differences in the total pressure evolution among theMELCORM3 default and BE cases

50000 100000 150000 2000000Time (s)

90

95

100

105

110

115

120

125

130

Atm

osph

eric

tem

pera

ture

(∘C)

EXPM3 defaultM3 BE

Figure 23 Differences in the atmospheric temperature evolution at232m among the MELCORM3 default and BE cases

values closer to the ones calculated with equation (1) andreported in Table 1 In Figures 22 23 and 24 the effect ofthe improved characteristic length value on the total pressurethe atmospheric temperature at 232m and the rh for theM3 model is reported The total pressure predictions sufferthe same uncertainties of the default case especially between90000 s and 100000 s On the contrary the atmospherictemperature and the rh during the aerosol washing andchemistry phases are now closer to the experimental valuesThe influence of these improvements on the aerosols andfission products behavior will be discussed in the followingsections

The conclusions that can be drawn from this sensitivityanalysis agree with those reported in [23] where the use of aCL value for a vertical plate-type HS equal to its real heightwas suggested for a good estimation of the condensationrate also if underpredicted heat transfer coefficients werestill reported although for other HSrsquos geometries no inves-tigations were reported in [23] and a clear advice that the

50000 100000 150000 2000000Time (s)

005

015

025

035

045

055

065

075

Relat

ive h

umid

ity (mdash

)

EXPM3 defaultM3 BE

Figure 24Differences in the relative humidity among theMELCORM3 default and BE cases

conclusions foundmay be not valid for other HSrsquos geometrieswas made

Under some points of view the present paper supportsthese conclusions even if only an investigation on the heattransfer coefficient was carried out (the condensation behav-ior was not studied because no condensation occurs in theconsidered hot HSs) Vertically oriented cylindrical HSs wereconsidered instead of vertical-plate HSs From the performedanalysis a CL value of few cmwas deemed necessary to betterreproduce the thermal-hydraulic transient of the PhebusFPT-0 test As stated in the MELCOR manual [21] the heattransfer regime is defined based on the ratio between theReynolds (Re) and theGrashof (Gr) numbersWhen the ratioRe2Gr is below 1 a natural convection regime is consideredand if above 10 a forced convection regime is establishedand an intermediate condition is considered in betweenConsidering the Re and Gr numberrsquos correlations ( (2) and(3)ndashin both equations the CL is expressed as L) it can beeasily noticed that both depend on theCL but with a differentmagnitude the Re number with a linear dependency and theGr number with a cubic-power dependency

Reynolds Number

Re =120588 sdot V sdot 119871120583

(2)

Grashof Number

Gr =119892 sdot 120573 sdot (119879

119904minus 119879infin) 1198713

]2(3)

This means thatndashkeeping constant all the otherparametersndashthe Gr decreases faster than the Re if lowerCL values are employed Thus the ratio Re2Gr will increaseas the considered CL value decreases meaning that thethermal-hydraulics conditions will move from a full naturalconvective regime to an intermediate or even to a full forcedconvection regime Because of this the calculated heat

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 15: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 15

ASTECMELCOR

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

MELCOR BEEXP

Figure 25 Suspended aerosol mass

transfer coefficient will increase as the employed CL valuedecreases Therefore this decrease of the CL value can beconsidered an effective way to increase the heat transfercoefficient predicted by MELCOR

8 Aerosols and Fission Products Results

As stated in the thermal-hydraulics results section onlyminor differences are shown among the results obtained withthe employed spatial nodalizations Therefore the aerosolsand FPs behaviors have been preliminarily investigatedemploying the three different models From this preliminaryanalysis it was found that only the third spatial nodalization(M3) is able to correctly predict the coupling between thethermal-hydraulics transient and the aerosols and FPs behav-ior For this reason in the following only the results obtainedwith the M3 model will be discussed All the chemicalelements reaching the containment vessel-as reported inthe final report [9]-have been simulated but the followingdiscussion will focus only on the main FPs and on structuralmaterials

In Figure 25 the aerosol mass suspended in the con-tainment atmosphere is reported During the first 23000 sonly four experimental data points were provided in [9] Thenumber of these pointsndashhighlighted in the graph with blackldquoXrdquo marks-is too scarce to closely follow the real evolution ofthe suspended aerosol massThe predictions of the two codesmay look quite poor in the considered time window but tworemarks have to be made

(i) For the second experimental point (18478 s) a noteis made in [9] highlighting that this value is notfully representative of the suspended aerosol masssince a loss of particles during the sampling processoccurred

(ii) The third and the fourth experimental points rep-resent the suspended aerosol mass at 21818 s and22568 s [9] Between these two measurements adifference of about 30 g is shown meaning that anabrupt deposition occurs in about 125min

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 26 Masses deposited onto the bottom containment vesselsurface before the washing phase

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

I

Mas

s (kg

)

ASTMEL

MEL BEEXP

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 27 Masses deposited onto the wet condenser surfaces andthe vertical walls after the washing phase

Considering the initial experimental data points except thesecond one both codes roughly match the experimentaldata A much smoother deposition behavior is shown byboth codes with the experimental ldquopeakrdquo at 21818 s beingreached at about 17000 s in the calculations The suspendedaerosol mass trends obtained with both codes are similar tothose reported in [25] even if in [25] maximum suspendedaerosol masses of 006 kg between 16000 s and 19000 s werereported

After 23000 s the experimental data points are sufficientto stress that both codes well match the experimental evolu-tion of the suspended aerosol mass

In Figures 26 and 27 the masses of the different ele-ments deposited onto the bottom containment vessel surfacebefore the washing phase and the masses deposited onthe wet condenser surfaces and the vertical walls after thewashing phase are reported The masses deposited on thebottom surface are well predicted by both codes Only minoruncertainties are shown for Ce and Zr probably becauseof their very low amount in containment On the contrarysignificant discrepancies are shown by MELCOR for the

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 16: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

16 Science and Technology of Nuclear Installations

0 100000 200000 300000

Mas

s (kg

)

Time (s)

AgI

CsIHIOEXP

1E minus 04

1E minus 05

1E minus 06

1E minus 07

1E minus 08

1E minus 09

1E minus 10

)2)minus

Figure 28 Iodine speciation in the sump water (MELCOR defaultcase)

masses deposited on the wet condenser surfaces and onthe vertical walls For ASTEC the only remark is relatedto the important overestimation of the Cs mass (one orderof magnitude) MELCOR provides quite poor results forthe deposited masses because aerosols on the condensersurfaces are captured only by the condensed water film(which flows into the sump water) This result is not due toa userrsquos assumption but it is the normal resulting behaviorof the water-film tracking model as explicitly declared bythe sensitivity coefficient no 7136 (solubility of RN classesin water films) [21] Such value could be changed by theuser but no sensitivity analyses have been performed due tothe absence of information about the repartition coefficientbetween the liquid film and the structure

The evolution of the masses for the different iodinespecies inside the sump is reported in Figures 28 and 29 forMELCOR and ASTEC respectively No appreciable differ-ences were shown between the two investigated MELCORcases (default and best estimate cases) so only the results forthe default case will be discussed in the following The mainaspects that should be stressed are as follows

(i) A significant difference exists in the total iodinemass flowing inside the sump water In ASTEC about35119864 minus 5 kg of iodine flows into the sump waterin MELCOR only 20119864 minus 6 kg The MELCOR resultis quite surprising because in the MELCOR modelthe aerosol mass deposited onto the wet condensersurfaces is completely routed toward the sump waterhence an overestimation of the iodine in the sumpwas expected Since the iodine mass deposited onthe bottom vessel surface is well captured the onlyexplanation for this lack of iodine mass in the sump isan overprediction of its presence in the atmosphereThe sump and the surrounding atmosphere exchangeiodine basing on the conditions established at theinterface of these two parts This poor prediction wasquite expected considering that the general modeliza-tion of the iodine chemistry in MELCOR does notreflect the actual state-of-the-art [26] In turn better

0 100000 200000 300000Time (s)

AgI HIO

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)minus )3minus

Figure 29 Iodine speciation in the sump water (ASTEC)

results are shown for ASTEC and a good agreementwith experimental data is shown

(ii) Iodine ismainly found in theAgI form for both codesThis result was expected considering the relevantamount of Ag released inside the containment vessel(005 kg) [26]

(iii) An important quantity of CsI is formed byMELCOROn the contrary CsI is not present in ASTEC becausethe code automatically selects the chemical reactionsthat can occur during the calculation performedFor a stand-alone containment analysis the reactionsinvolving Cs and Iodine are not considered becausethe containment thermal-hydraulics conditions pre-vent the formation of CsI This assumption is auto-matically made by the code and it is based on theactual state-of-the-art on iodine chemistry [26] Forthis reason the formation of CsI in MELCOR isunrealistic and it is due to the actual poor iodinemodelling implemented in MELCOR which mainlyreflects the conclusions reported in [27]

(iv) I2 Iminus HIO and IO

3

minus behaviors are completely dif-ferent for the two codes In MELCOR an importantfraction of iodine is also found under Iminus HIO andI2forms In ASTEC no I

2is found inside the sump

while small amounts of HIO and IO3

minus are shown Inturn Iminus is rapidly dissolved and converted to otherchemical forms (mainly AgI) once entered in thesump Iminus then reappears during the washing phaseand the precedent preparatory actions but it is rapidlydissolved again once ending the washing phase Inany case the iodine behavior shown by ASTEC agreesquite well with the actual state-of-the-art [26]

In general the aerosol and FP predictions are not completelysatisfactory because of the too low iodinemass flowing insidethe sump Regarding the speciation in the sump MELCORshows an important amount of AgI and CsI during thedegradation and the aerosol phases then CsI is partiallydissociated into I

2during the washing and the chemistry

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

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Submit your manuscripts athttpswwwhindawicom

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Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 17: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 17

1020 (default)

15000 20000 25000 30000 35000 4000010000Time (s)

0

001

002

003

004

Mas

s (kg

)

30EXP

Figure 30 Results of the MELCOR sensitivity analyses onNPSCndashsuspended aerosol mass

phases In ASTEC iodine is found mainly under the AgIform for the entire test and for short periods also underthe Iminus form Of these two behaviors the ASTEC one is themost reliable one since it reflects the actual state-of-the-arton iodine chemistry in a sump [26]

9 Aerosols and Fission ProductsSensitivity Analyses

The influence of different input parameters has been investi-gated through a quite large execution of sensitivity analysesIn the following the findings of these analyses will bebriefly discussed Regarding the two MELCOR M3 casesinvestigated (default and best-estimate) minor differenceswere shown so in the following only the results of the M3default case will be reported

The main input parameters investigated for both codeswere as follows

(i) Number of Particle Size Classes (NPSC) This parameterhas no physical meaning but it is the number of intervals(classes) in which the log-normal distribution of the injectedaerosols is subdivided A default value of 10 is suggested forMELCOR while for ASTEC no default values are provided[21] In the present paper five sensitivity cases have beeninvestigated setting this parameter to 10 20 (default case)30 40 and 50 The cases with NPSC sets to 20 30 40and 50 have provided identical results while NPSC sets to10 minor differences were shown for the suspended aerosolmass In Figures 30 and 31 the influence of this parameteron the suspended aerosol mass in MELCOR and ASTECcalculations is reported Hence the results highlight that thisparameter has only a minor influence on the overall aerosolsand FPs behavior With a NPSC set to 10 slightly lowercomputation times are required to perform the calculationbut the difference is not so marked

(ii) Dynamic Shape Factor for Agglomeration Processes Adefault value of 10 is suggested for both codes [18 21]

1020 (Default)30

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

4050EXP

Figure 31 Results of the ASTEC sensitivity analyses onNPSCndashsuspended aerosol mass

Four sensitivity analyses have been performed setting thisparameter to 10 (default case) 15 20 and 30 A value of 10means that the aerosol particles are spherical 15 means thatare similar to sand and 20 means that they are similar to talc[28] This parameter has a great influence on the suspendedaerosol mass evolution in both codes but only minor effectsare shown among the deposition repartition among thedifferent surfaces Increasing the dynamic shape factor slowsthe deposition processes (because the agglomeration is lessstrong) leading to results not comparable with the experi-mental ones Regarding the deposition repartition among thedifferent surfaces scattered effects are shown In ASTEC theincrease of the dynamic shape factor increases the depositiononto the vertical walls and on the wet condenser surfaceswhile in MELCOR different behaviors are shown for eachelement For Cs Te Ru and Re the deposition on the verticalwalls and on the wet condenser surfaces is stronger for BaSr Cd and Sb it is weaker and for U Ag Sn In and I

2no

appreciable differences were shown In Figures 32 33 34 and35 the influence on the suspended aerosol mass evolutionand on the mass deposited onto the vertical walls and the wetcondenser surfaces is shown

(iii) Aerosol Density No experimental data nor default valueswere provided for the aerosol density but in other worksvalues spanning from 1000 kgm3 to 10000 kgm3 were sug-gested [29 30] In the present paper five sensitivity cases wereinvestigated setting the density value to 1000 kgm3 (defaultMELCOR value for wet aerosols [21]) 3000 kgm3 (defaultcase) 10000 kgm3 15000 kgm3 and 20000 kgm3 Thedefault aerosol density assumed (3000 kgm3) was inferredfrom previous works [29 30] The increase of the aerosoldensity leads to a faster deposition onto the vertical walls inMELCOR and on the bottom surface inASTECThe increaseof the aerosol density obviously decreases also the maximumaerosol mass suspended inside the containment due to thefaster deposition process In Figures 36 37 38 and 39 theeffects of density variation on the suspended aerosol mass

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 18: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

18 Science and Technology of Nuclear Installations

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 32 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashsuspended aerosol mass

0

001

002

003

004

005

006

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

10 (default)1520

30EXP

Figure 33 Results of theASTEC sensitivity analyses on the dynamicshape factorndashsuspended aerosol mass

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 34 Results of the MELCOR sensitivity analyses on thedynamic shape factorndashdeposited aerosol mass on the vertical wallsand on the wet condenser surfaces

10 (default)1520

30EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 35 Results of theASTEC sensitivity analyses on the dynamicshape factorndashdeposited aerosol mass on the vertical walls and on thewet condenser surfaces

10003000 (default)10000

1500020000EXP

00005

0010015

0020025

0030035

0040045

005M

ass (

kg)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 36 Results of the ASTEC sensitivity analyses on the aerosoldensityndashsuspended aerosol mass

and on the deposition onto the vertical walls and on the wetcondenser surfaces are shown

(iv) Agglomeration Shape Factor A default value of 10 issuggested for both codes [18 21] Four sensitivity analyseshave been executed setting this parameter equal to 05 10(default case) 20 and 30 The effects on the ASTEC andMELCOR calculations are somewhat similar to those shownwith an increased aerosol density In ASTEC the increase ofthe agglomeration shape factor leads to a greater depositionon the bottom surface while in MELCOR no remarkableeffects on the deposition process onto the containmentsurfaces are shown because a greater fraction of the aerosolmass is transported into the sump In Figures 40 and 41 theeffects of this parameter on the suspended aerosol mass areshown

(v) Turbulence Dissipation Rates In ASTEC a default valueof 002m2s3 is suggested [18] while in MELCOR the

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 19: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 19

10003000 (default)10000

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

1500020000EXP

Figure 37 Results of the MELCOR sensitivity analyses on theaerosol densityndashsuspended aerosol mass

) 2

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr

Ru +

ReTe U

Ce +

Zr

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

Figure 38 Results of the ASTEC sensitivity analyses on the aerosoldensityndashdeposited aerosol mass on the vessel vertical walls and onthe wet condenser surfaces

10003000 (default)10000

1500020000EXP

Cs

Ba +

Sr Te

Ru +

Re

Ce +

Zr U

Cd

+ Sb

Ag

+ In

+Sn

Mas

s (kg

)

1E minus 01

1E minus 03

1E minus 05

1E minus 07

1E minus 09

) 2

Figure 39 Results of the MELCOR sensitivity analyses on theaerosol densityndashdeposited aerosol mass on the vessel vertical wallsand on the wet condenser surfaces

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 40 Results of the MELCOR sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

0510 (default)20

30EXP

Figure 41 Results of the ASTEC sensitivity analyses on theagglomeration shape factorndashsuspended aerosol mass

default value is set to 0001m2s3 [21] Five sensitivityanalyses have been executed setting this parameter to0001m2s3 0005m2s3 001m2s3 002m2s3 (default case)and 003m2s3 In ASTEC no appreciable differences wereshown among the different cases while in MELCOR dif-ferences exist on the suspended aerosol mass but not onthe deposited mass amount on the different surfaces InFigure 42 the effects of this parameter on the MELCORsuspended mass are shown

(vi) Particle Sticking Probability A default value of 10 issuggested for both codes [18 21] Four sensitivity cases havebeen investigated setting this parameter to 05 10 (defaultcase) 20 and 30 A strong influence on the suspendedaerosol mass is shown in MELCOR but no remarkabledifferences on the deposition magnitude on the differentcontainment surfaces were shown In ASTEC the influenceis lower than that in MELCOR but again no differences wereshown on the magnitude for the deposition on the different

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 20: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

20 Science and Technology of Nuclear Installations

10000 15000 20000 25000 30000 35000 40000Time (s)

00010005001

002 (default)003EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

Figure 42 Results of the MELCOR sensitivity analyses on theturbulence dissipation ratendashsuspended aerosol mass

0510 (default)2

3EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 43 Results of the ASTEC sensitivity analyses on the particlesticking probabilityndashsuspended aerosol mass

containment surfaces In Figures 43 and 44 the effects of thisparameter on the suspended aerosol mass are reported

(vii) Ratio of theThermal Conductivity of the Gas Phase to theThermal Conductivity of the Aerosol Particles Defaults valuesof 50119864 minus 2 and 50119864 minus 3 are suggested for MELCOR andASTEC respectively [18 21] Four sensitivity cases have beenperformed setting this parameter to 50119864 minus 2 (default case)50119864minus3 50119864minus4 and 50119864minus5 In both codes the different casesinvestigated have provided the same results so the influenceof this parameter seems negligible

(viii) Aerosol Mass Median Diameter (AMMD) and ItsGeometric Standard Deviation (GSD) A maximum and aminimum AMMD and GSD values for the entire aerosolpopulation were provided in the FPT-0 Final Report [9] Forthis reason three sensitivity analyses were carried out settingAMMD to 09 120583m 13120583m(default case) and 17 120583m andGSDto 20 215 (default case) and 23 The modification of the

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

0510 (default)20

30EXP

Figure 44 Results of the MELCOR sensitivity analyses on theparticle sticking probabilityndashsuspended aerosol mass

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 45 Results of the ASTEC sensitivity analyses onAMMDndashsuspended aerosol mass

GSD value provided negligible results On the contrary agreater influence for AMMD was highlighted Such effectsare mainly focused on the suspended aerosol mass evolutionbut not for the magnitude of the deposition process onthe different containment surfaces In Figures 45 and 46the effects of AMMD on the suspended aerosol mass arereported

Both codes implement other specific input parametersSome of these parameters were investigated but a code-to-code comparisonwas not executedThese specific parametersfor ASTEC were as follows

(i) Influence of the Aerosol Particles on the Gas Density Theinfluence of the aerosol particles on the gas density has anegligible effect on the ASTEC results

(ii) Flag to Dynamically Calculate the Condensation TimeStep Five cases were investigated changing the surfacetension of a droplet in the atmosphere in the default case

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 21: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 21

0913 (default)

17EXP

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 46 Results of the MELCOR sensitivity analyses onAMMDndashsuspended aerosol mass

the condensation time step was not dynamically calcu-lated while in the other cases such time step was dynam-ically calculated considering four different droplet surfacetension values (00586Nm 00598Nm 00572Nm and00559Nm) These four values were calculated consider-ing different droplet equilibrium temperature values 100∘C(00586Nm) 95∘C (00598Nm) 105∘C (00572Nm) and110∘C (00559Nm) No differences were shown among theresults of the investigated cases

(iii) Air Molecular Weight A default value of 235 kgkmolis suggested based on the common air composition but inthe present scenario the containment vessel atmosphere ismainly filled with N

2 so a value closer to the N

2molecu-

lar weight (2896 kgkmol) may be more appropriate Foursensitivity cases have been performed setting this parameterto 235 kgkmol (default case) 2896 kgkmol (atmosphereonly filled with N

2) 1802 kgkmol (atmosphere only filled

with O2) and 2076 kgkmol (atmosphere filled with 50 N

2

and 50 O2) Negligible differences were shown among the

results of the investigated cases

(iv) Relation Employed to Calculate the Collision Efficiency forGravitational andTurbulent Coagulation Therecommendedmodel is the Pruppacher and Klett one but the Fuchsmodel and the truncated Pruppacher and Klett model areimplemented in the ASTEC code as well For this reasonthree sensitivity cases have been performed one for eachimplemented model A minor difference on the suspendedaerosol mass among the Pruppacher and Klett models (both)and the Fuchs model was highlighted although the threemodels predict values higher than the experimental ones atleast until 25000 s In Figure 47 the entity of this differencefor the suspended aerosol mass is shown

(v) Thickness of the Water Film Used for Drainage andAerosol Washdown No default values are suggested so threesensitivity cases have been performed setting this parameterto 10119864 minus 3m 10119864 minus 4m (default case) and 10119864 minus 5m No

00EXP

15000 20000 25000 30000 35000 4000010000Time (s)

00005

0010015

0020025

0030035

0040045

005

Mas

s (kg

)

minus20

minus10 (default)

Figure 47 Results of the ASTEC sensitivity analyses on the relationemployed to calculate the collision efficiency for gravitational andturbulent coagulationndashsuspended aerosol mass

appreciable differences have been highlighted among thesethree cases

(vi) Aerosol Deposition Velocity for Each Aerosol ComponentNo default values are suggested so four sensitivity cases havebeen executed setting this parameter to 00ms (default case)10119864 minus 2ms 10119864 minus 3ms and 10119864 minus 4ms No appreciabledifferences were found among these cases

For MELCOR these specific input parameters wereinvestigated

(i) Condensation of Water on All the Aerosol Particles or Onlyon Aerosol Particles Containing Water Two sensitivity caseshave been analyzed one setting the condensation on all theaerosol particles (default case) and the other one setting thecondensation only on the aerosol particles containing waterNo differences were shown among the two cases

(ii) Particle Slip Coefficient Influencing the GravitationalDeposition [21] A default case of 1257 is suggested in theMELCORUserrsquos Guide [21] but three other values have beeninvestigated setting this parameter to 10 15 and 20 Minordifferences were shown for the deposition magnitude on thevertical walls and on the wet condenser surfaces

(iii) Constant Associated with the Thermal AccommodationCoefficient for theThermophoresis DepositionMechanism [21]A default value of 225 (dimensionless) is suggested in theMELCOR Userrsquos Guide [21] but four other cases have beenperformed setting this parameter to 10 20 25 and 30As for the previous parameter only minor differences wereshown for the depositionmagnitude on the vertical walls andon the wet condenser surfaces

(iv) Diffusion Boundary Layer Thickness A default value of10119864 minus 5m is suggested in the MELCOR Userrsquos Guide [21]but four other cases have been executed setting this valueto 10119864 minus 4m 50119864 minus 4m 50119864 minus 5m and 10119864 minus 6m In

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 22: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

22 Science and Technology of Nuclear Installations

1E-45E-41E-5 (default)

5E-51E-6EXP

0

001

002

003

004

Mas

s (kg

)

15000 20000 25000 30000 35000 4000010000Time (s)

Figure 48 Results of theMELCOR sensitivity analyses on diffusionboundary layer thicknessndashsuspended aerosol mass

a previous work [31] a value of 10119864 minus 4m was suggestedWith a diffusion boundary layer thickness set to 10119864 minus 6 thedeposition process velocities are slightly overestimated and atoo fast deposition is shown although the other values for thediffusion boundary layer thickness provided identical resultsso it seems that this parameter has a poor influence on theaerosol behavior In Figure 48 the effects on the suspendedaerosol mass are reported

(v) Partition of Iminus and HIO between Atmosphere and SumpIn the default case the partitioning for both componentsis deactivated (as suggested in the Userrsquos Guidelines [21])but two sensitivity cases have been performed selectivelyactivating the Iminus and the HOI partitioning The activation ofIminus partitioning led to a quite important Iminus release from thesump water towards the atmosphere influencing the iodinespeciation As shown in Figure 49 the same iodine species ofthe default case are formed but themasses produced aremorethan one order of magnitude lower than the default case ones(except AgI) Also the HIO partitioning has a great effecton the sump speciation during and after the washing phase(Figure 50) After the washing phase the HIO partitioningand the Iminus partitioning show quite similar result

10 Conclusions

In the present paper a comparison between the ASTECand MELCOR codes against the results of the Phebus FPT-0 test has been executed Three spatial nodalizations havebeen employed to stress that too simple nodalizations arenot able to correctly simulate a complex thermal-hydraulictransient These nodalizations have been also developed inthe most identical way possible for both codes but thedifferent modelling approaches of certain aspects were allexploited to obtain the best possible results with each code

The most complex spatial nodalization (M3) had showngood thermal-hydraulic results but some discrepancies were

AgIHIO

CsI

EXP

200000100000 3000000Time (s)

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

)2

)minus

Figure 49 Effect of Iminus partitioning on the iodine atmospheric mass

AgIHIO

CsI

EXP

1E minus 10

1E minus 09

1E minus 08

1E minus 07

1E minus 06

1E minus 05

1E minus 04

Mas

s (kg

)

100000 200000 3000000Time (s)

)2

)minus

Figure 50 Effect of HOI partitioning on the iodine atmosphericmass

shown for both codes These discrepancies are mainly intro-duced by the performed userrsquos assumptions A clear exampleof these effects is shown during the time period of thepreparatory actions before the washing phase and during thebeginning of the chemistry phase Such effects are mainlyfocused on the atmospheric temperature predictions whichin turn influence the total containment pressure and therh results although the aerosolsrsquo behavior is mainly influ-enced by the thermal-hydraulics conditions during the early50000 s of the test but also the thermal-hydraulic transientduring the other test phases becomes important for a correctevaluation of the iodine behavior in the sumpwater and in theatmosphere Few experimental data were provided inside theFPT-0 Final Report [9] but some conclusions can be drawn

(i) Probably during the early FP injection phase thecalculated suspended aerosol mass is overestimatedHowever only two experimental data points wereprovided during this period so this conclusion can-not be completely ensured

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 23: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 23

Table 4 Summary of the sensitivity cases performed and importance for the ASTEC and MELCOR results

Parameter Importance for ASTEC calculations Importance for MELCOR calculationsIminus partitioning among sump water and atmosphere HighHIO partitioning among sump water and atmosphere HighAerosol deposition velocity for each aerosol component Low

Dynamic shape factor for agglomeration processes High HighCollision efficiency for gravitational and turbulentcoagulation High

Aerosol density High HighDynamic calculation of the time-step + Surface tensionof a droplet in the atmosphere Low

Agglomeration shape factor High HighConsideration of the aerosol particles presence forcalculating the gas density Low

Number of particle size classes High HighAerosol mass median diameter High HighGeometric standard deviation Low LowParticle sticking probability High HighThickness of water film used for drainage and aerosolwashdown Low

Thermal conductivity of gas divided by thermalconductivity of aerosol particle Low Low

Turbulence dissipation rate Minor HighMolecular weight of gas Minor

Diffusion boundary layer thickness MinorThermal accommodation coefficient MinorParticle slip coefficient MinorCondensation onto wet aerosols Low

(ii) The MELCOR default and best estimate cases predictslightly different aerosol behavior but such differ-ences are of minor importance

(iii) The magnitude of the deposition processes is wellcaptured by ASTEC while for MELCOR somewhatpoor results are shown especially for the depositiononto the vertical walls and on the wet condensersurfaces although these results are mainly due to theparticularmodels implemented in theMELCOR code(the FPs deposited onto a structure are completelywashed away if a condensate film exists on its surface)

(iv) The performed sensitivity analyses have shown thatseveral parameters influence the aerosol behaviorThemost influencing parameters were found to be theaerosol density the dynamic agglomeration factorthe agglomeration factor and the AMMD Otherparameters slightly influencing the code predictionsare the sticking probability the number of particle sizeclasses the turbulence dissipation rate the relationemployed to calculate the collision efficiency for grav-itational and turbulent coagulation (only in ASTEC)the air molecular weight (only in ASTEC) the diffu-sion boundary layer thickness (only inMELCOR) theparticle slip coefficient influencing the gravitational

deposition (only inMELCOR) and the constant asso-ciated with the thermal accommodation coefficientfor the thermophoresis deposition mechanism (onlyin MELCOR) Other investigated parameters witha negligible influence were the injection GSD thethermal conductivity of gas divided by thermal con-ductivity of aerosol particle the aerosol depositionvelocity (only in ASTEC) the dynamic calculationof the condensation time step (only in ASTEC) thethickness of water film used for drainage and aerosolwashdown (only in ASTEC) the consideration ofthe aerosol particles presence for calculating the gasdensity (only in ASTEC) and the aerosol condensa-tion on all aerosols or only on wet aerosols (only inMELCOR) In Table 4 a summary of the sensitivitycases performed is reported

Quite important differences were shown about the predictionof the iodine behavior in the sump Both codes-but especiallyMELCOR-fail to catch the correct repartition among thedeposited iodine on structures the iodine inside the sumpwater and the suspended iodine mass This poor estimationmay be also one of themain causes behind the different iodinespeciation in the sump water In the ASTEC simulationsiodine is found mainly under AgI and Iminus forms in MELCORanalysis in AgI CsI and I

2 Other species are also formed

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 24: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

24 Science and Technology of Nuclear Installations

as HIO (both codes) and IO3

minus but their amounts are almostnegligible The ASTEC predictions seem the most realisticones thanks to the important formation of AgI and theabsolute absence of CsI A sensitivity analysis has been alsocarried out inMELCOR for the evaluation of the influence ofthe activationdeactivation of the Iminus and HOI partitioning onthe iodine behavior in the sumpThese cases have highlightedthat the partition of both compounds leads to quite differentresults so their activation should be carefully evaluated oncase by case basis

Nomenclature

120573 Thermal expansion coefficient [1K]120583 Dynamic viscosity [Pasdots]] Kinematic viscosity [m2s]120588 Density [kgm3]119892 Gravity acceleration [ms2]119871 Characteristic length [m]119879119904 Surface temperature [K]119879infin Bulk temperature [K]

V Flow velocity [ms]

Conflicts of Interest

The authors declare that they have no conflicts of interest

References

[1] J-P Van Dorsselaere A Auvinen D Beraha et al ldquoRecentsevere accident research synthesis of the major outcomes fromthe SARNET networkrdquo Nuclear Engineering and Design vol291 pp 19ndash34 2015

[2] A Bentaıb H Bonneville G Cenerino et al Nuclear PowerReactor Core Melt Accidents ESP Sciences 2015

[3] B Clement and R Zeyen ldquoThe objectives of the Phebus FPexperimental programme andmain findingsrdquoAnnals of NuclearEnergy vol 61 pp 4ndash10 2013

[4] O De Luze T Haste M Barrachin and G Repetto ldquoEarlyphase fuel degradation in Phebus FP Initiating phenomena ofdegradation in fuel bundle testsrdquo Annals of Nuclear Energy vol61 pp 23ndash35 2013

[5] T Haste F Payot and P D W Bottomley ldquoTransport anddeposition in the Phebus FP circuitrdquo Annals of Nuclear Energyvol 61 pp 102ndash121 2013

[6] M Laurie P March B Simondi-Teisseire and F Payot ldquoCon-tainment behaviour in Phebus FPrdquo Annals of Nuclear Energyvol 60 pp 15ndash27 2013

[7] P March and B Simondi-Teisseire ldquoOverview of the facilityand experiments performed in Phebus FPrdquo Annals of NuclearEnergy vol 61 pp 11ndash22 2013

[8] G Brillant C Marchetto and W Plumecocq ldquoFission productrelease from nuclear fuel II Validation of ASTECELSA onanalytical and large scale experimentsrdquo Annals of NuclearEnergy vol 61 pp 96ndash101 2013

[9] N Hanniet-Girault and G Repetto FPT-0 Final Report Institutde Radioprotection et de Surete Nucleaire Saint-Paul-Les-Durance France

[10] G Gyenes and L Ammirabile ldquoContainment analysis onthe PHEBUS FPT-0 FPT-1 and FPT-2 experimentsrdquo NuclearEngineering and Design vol 241 no 3 pp 854ndash864 2011

[11] K Mueller S Dickinson C de Pascale et al ldquoValidation ofsevere accident codes on the phebus fission product tests in theframework of the PHEBEN-2 projectrdquo Nuclear Technology vol163 no 2 pp 209ndash227 2008

[12] V D Layly P Spitz S Tirini and A Mailliat ldquoAnalysis of thephebus FPT0 containment thermal hydraulics with the Jerichoand Trio-VF codesrdquo Nuclear Engineering and Design vol 166no 3 pp 413ndash426 1996

[13] F d Rosa and R Mari ldquoThermal-hydraulics and physics nearPhebus condenser a comparison between codes and againstsome experimental data from FPT0 testrdquo in Proceedings of the7th International Conference on Nuclear Engineering (ICONE)Tokyo Japan 1999

[14] A Bujan G Gyenes and H Wider Final Interpretation Reporton the FPT0 and FPT1 Circuit and Containment Analyses 2007

[15] M Di Giuli T Haste R Biehler et al ldquoSARNET benchmarkon Phebus FPT3 integral experiment on core degradation andfission product behaviourrdquoAnnals of Nuclear Energy vol 93 pp65ndash82 2016

[16] B Gonfiotti and S Paci ldquoStand-alone containment analysisof the Phebus FPT-1 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 2016 24th International Conferenceon Nuclear Engineering ICONE 2016 Charlotte NC USA June2016

[17] A Kontautas and E Urbonavicius ldquoAnalysis of aerosol depo-sition in PHEBUS containment during FPT-1 experimentrdquoNuclear Engineering and Design vol 239 no 7 pp 1267ndash12742009

[18] W Klein-Hessling and B Schwinges ASTEC V0 CPA Mod-ule Program Reference Manual nstitut de Protection et deSurete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reack-torsicherheit (GRS) mbH 1998

[19] L Bosland ASTEC V20 rev2 Code IODE Module Iodine AndRuthenium Behaviour in The Containment Institut de Protec-tion et de Surete Nucleaire (IPSN)Gesellschaft fur Anlagen-und Reacktorsicherheit (GRS) mbH Saint Paul-lez-DuranceFrance 2011

[20] G Guillard F Jacq C Seropian and W Plumecocq ASTECV1 Code SYSINT Module Management of Events and Safety Sys-tems Interactions Institut de Protection et de Surete Nucleaire(IPSN)Gesellschaft furAnlagen-undReacktorsicherheit (GRS)mbH Saint Paul-lex-Durance France 2007

[21] L L Humphries R K Cole D L Louie V G Figueroa andMF YoungMELCOR Computer Code Manuals Vol1 Primer andUserrsquos Guide Sandia National Laboratory 2015

[22] J L Tills A Notafrancesco and J Phillips Application of theMELCOR Code to Design Basis PWR Large Dry ContainmentAnalysis Sandia National Laboratories Albuquerque NMUSA 2009

[23] D S YoonH Jo andM L Corradini ldquoAssessment ofMELCORcondensation models with the presence of noncondensable gasin natural convection flow regimerdquo Nuclear Engineering andDesign vol 317 pp 110ndash117 2017

[24] B Gonfiotti and S Paci ldquoA stand-alone containment analysisof the Phebus FPT-0 test with the ASTEC and the MELCORcodesrdquo in Proceedings of the 8th conference on Severe AccidentResearch ERMSAR 2017 Warsaw Poland 2017

[25] A V Jones S Dickinson C De Pascale et al ldquoValidation ofsevere accident codes against Phebus FP for plant applications

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 25: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

Science and Technology of Nuclear Installations 25

Status of the PHEBEN2 projectrdquo Nuclear Engineering andDesign vol 221 no 1-3 pp 225ndash240 2003

[26] B Clement L Cantrel G Ducros et al ldquoState of the Art Reporton Iodine Chemistryrdquo Tech Rep 2007

[27] L Soffer S Burson C Ferrell R Lee and J Ridgely ldquoAccidentSource Terms for Light-Water Nuclear Power Plantsrdquo TechRep US Nuclear Regulatory Commision Washington WAUSA 1995

[28] W C Hinds Aerosol Technology Wiley-Interscience Publica-tion 2012

[29] E Hontanon J Polo and L E Herranz ldquoOn the modellingcapabilities to simulate aerosol behaviour in the PHEBUS-FP containment Lessons learned from FPT0 testrdquo Journal ofAerosol Science vol 27 no 1 pp S459ndashS460 1996

[30] P Dumaz and S Cho ldquoThermophoretic deposition in the firstPhebus-FP experimentrdquo Journal of Aerosol Science vol 26 pp83-84 1996

[31] A Kontautas E Babilas and E Urbonavicius ldquoCOCOSYSanalysis for deposition of aerosols and fission products inPHEBUS FPT-2 containmentrdquoNuclear Engineering and Designvol 247 pp 160ndash167 2012

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Page 26: Stand-Alone Containment Analysis of the Phébus FPT Tests with …downloads.hindawi.com/journals/stni/2017/1450648.pdf · 2019-07-30 · ResearchArticle Stand-Alone Containment Analysis

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal of

Volume 201Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014