steam generator tube rupture (sgtr)...

Nuclear Engineering and Design 235 (2005) 457–472

Steam generator tube rupture (SGTR) scenarios

A. Auvinena, J.K. Jokiniemia,∗, A. Lahdea, T. Routamob, P. Lundstromb,H. Tuomistob, J. Dienstbierc, S. Guntayd, D. Suckowd, A. Dehbid, M. Slootmane,

L. Herranzf, V. Peyresf, J. Polof

a VTT Processes, Biologinkuja 7, P.O. Box 1602, VTT Espoo 02044, Finlandb Fortum Nuclear Services, Vantaa, Finland

c Nuclear Research Institute Rez plc, Czech Republicd Paul Scherrer Institute, Villigen-PSI, Switzerland

e Nuclear Research and Consultancy Group, Arnhem, Netherlandsf Centro de Investigaciones Energ´eticas, Medioambientales y Tecnol´ogicas, Madrid, Spain

Received 30 March 2004; received in revised form 6 May 2004; accepted 31 August 2004

Abstract

The steam generator tube rupture (SGTR) scenarios project was carried out in the EU 5th framework programme in thefield of nuclear safety during years 2000–2002. The first objective of the project was to generate a comprehensive databaseo to supporta re accidentc ct retentionw , which ist ults of thep horizontals©

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a

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bingthattial

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n fission product retention in a steam generator. The second objective was to verify and develop predictive modelsccident management interventions in steam generator tube rupture sequences, which either directly lead to seveonditions or are induced by other sequences leading to severe accidents. The models developed for fission produere to be included in severe accident codes. In addition, it was shown that existing models for turbulent deposition

he dominating deposition mechanism in dry conditions and at high flow rates, contain large uncertainties. The resroject are applicable to various pressurised water reactors, including vertical steam generators (western PWR) andteam generators (VVER).2004 Elsevier B.V. All rights reserved.

. Introduction and research objectives

Steam generator (SG) reliability and performancere serious concerns in the operation of pressurised

∗ Corresponding author. Tel.: +358 9 456 6158;ax: +358 9 456 7021.

E-mail address:[email protected] (J.K. Jokiniemi).

water reactors. In particular, steam generator tuis subject to a variety of degradation processescan lead to tube cracking, wall thinning, and potenleakage or rupture (MacDonald et al., 1996). Over thelast decade, a considerable effort has been speunderstand these degradation processes and to imrelated preventive and corrective actions as weoperational aspects. However, steam generator

029-5493/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.nucengdes.2004.08.060

458 A. Auvinen et al. / Nuclear Engineering and Design 235 (2005) 457–472

Nomenclature

AM accident managementAMMD aerodynamic mass median diameterBC base caseBLPI Berner low-pressure impactordt tube diameterD tube equivalent diameterDF decontamination factorECCS emergency core cooling systemEFW early feedwater recoveryEOP emergency operation procedureFBG fluidised bed generatorFW feedwaterGSD geometric standard deviationMSGTR multiple steam generator tube ruptureNbins number of particle binsNtubes number of tubes over which deposition

is consideredNC non-condensableNPP nuclear power plantOPC optical particle counterp absolute pressurepcold pressure in cold collectorphot pressure in hot collectorPSA probabilistic safety assessmentPWR pressurised water reactorRe Reynolds numbers distance between tubes

ηimpST single tube inertial impaction efficiency

ln(σg) logarithm of the geometric standard de-viation

leakage incidents have proven that such occur-rence cannot be completely ruled out. If a steamgenerator tube ruptures during a severe accident,radionuclides may leak from primary circuit to thesecondary side and bypass the containment. Accordingto most probabilistic safety assessment (PSA) studies,a significant fraction of fission products are assumedto flow through an unisolated break in an SG. Theassumption is based on an expert elicitation panel,since no experimental data of this phenomenon isavailable to verify it (USNRC, 1990).

General knowledge on retention of fission productsin the steam generator tubes and in the secondaryside was poor at the beginning of the project. Mostprevious experimental programs have concentratedon the initial stages of deposition inside tubes. Muchless attention has been paid to situations where thedeposition/resuspension/revaporisation changes asthe deposit layers build up as occurs under expectedaccident conditions (Wright, 1994). The understandingof fission product retention under realistic steam gen-erator conditions is needed in order to design efficientaccident management procedures. This is consideredvery important, since steam generator tube rupturesare included in the risk-dominant sequences. Thus, thefirst objective of the SGTR project was to generate acomprehensive database on fission product retention ina steam generator. The second objective was to verifyand develop predictive models to support accident

SEM scanning electron microscopeSG steam generatorSGCB steam generator collector breakSGTR steam generator tube ruptureSRV safety relief valveStk Stokes numberTEOM tapered element oscillating microbal-

anceVVER Russian-type pressurised water reactorWP work packagey(k) mass fraction of particles in size classk

Greek lettersηST(i, k) single tube retention efficiency for par-

ticle sizekηtbt

ST single tube turbulent deposition effi-ciency

management interventions during SGTR sequences.re de-

s SG.C dingo aters m ina ucts.T termi ap-p it. As e ef-f tions

The severe accident management procedures aigned to minimise the release from the defectedurrent accident management actions foresee floof the secondary side through the emergency feedwystem and depressurisation of the primary systen attempt to suppress the release of fission prodhese actions may significantly reduce the source

n SGTR of accidents. However, there has been noropriate database or associated model to estimatetrategic goal of the project was to demonstrate thectiveness of the accident management interven

A. Auvinen et al. / Nuclear Engineering and Design 235 (2005) 457–472 459

in reducing the source term even for severe accidentsthat lead to a bypass of the containment. The results ofthe project are applicable to various pressurised waterreactors, including vertical steam generators (westernPWR) and horizontal steam generators (VVER).

2. Work programme

The project was carried out as parallel studies forvertical and horizontal steam generators. The work wasdivided into four work packages (WP). The descriptionof each WP is given below.

The work in WP1 included the definition of impor-tant steam generator tube rupture sequences for the ver-tical steam generators (SG) of PWR and the horizontalSG of VVER-440 and determining the experimentalconditions.

The experimental investigations (WP2) were di-vided into integral and separate effect studies. The in-tegral experiments for vertical steam generators wereconducted at PSI in the ARTIST facility, which is ascaled-down model of the Framatome type SG in op-eration at the Swiss NPP Beznau 1136 MWth PWR.HORIZON facility at Fortum was used in the integralstudies of horizontal steam generators. The facility isa scaled-down model of VVER-440 steam generatorinstalled in Loviisa NPP (Finland) and Dukovany NPP(Czech Republic). Aerosol retention mechanisms act-ing at the secondary side near the break exit were stud-i na wass c-t ffectd man-a soci-a

si-c tionp f theS ichc itionp encesa osolc

elsd d tos ses.

In addition to the study of aerosol retention, sensitivitycalculations should assess the effectiveness of differentaccident management measures in the reference plants.

Two state of the art reports were also a part of thework. The first one described the present knowledgeof aerosol deposition mechanisms in tubes and tubebundle. The second report was an overview of presentstatus of SGTR events and procedures in the designbasis and beyond design basis domains for the relevantpower plants.

3. Work performed and results obtained

3.1. State of the art report

The state of the art in SGTR accidents was writ-ten in two reports (Jokiniemi et al., 2002; Bakker andSlootman, 2002). The first report describes the mostimportant deposition mechanisms in SGTR cases. Itwas concluded that even though the deposition mecha-nisms acting on SGTR scenarios are known, their mag-nitude and importance in different SGTR conditions isnot understood. Thus, experimental data were neededto justify the relevant deposition mechanisms and theirmagnitude.

The second report gives a general description ofSGTR events both within design basis and beyond de-sign basis situations. The international projects and lit-erature on the SGTR phenomena is discussed in ther nceo andD OPa andD areb maina andt ovi-i

3(

SGt maino ex-p Ne ition

ed in PECA facility at CIEMAT. Aerosol depositiond resuspension in the primary side of the SGtudied in PSAERO facility at VTT. The main objeive in WP2 was to develop integral and separate eatabases to assess the capability of the accidentgement measures, and to develop and verify asted mathematical models.

The objective of the WP3 was to develop phyal models capable of predicting the local deposihenomena in the primary and secondary sides oG. The aim was to build up a simple model, whould predict the experimentally observed deposhenomena in steam generator tube rupture sequs a function of flow rate, tube rupture size, aeroncentration, aerosol size, etc.

The objective of the WP4 was to include the modeveloped in WP3 into the system level codes, anee their effect on the aerosol retention in SGTR ca

eport. The present way of handling the occurref SGTR events within Borssele, Beznau, Loviisaukovany NPPs is also reviewed. This includes End SAM responses. For the Borssele, Beznauukovany NPPs, the EOP and SAMG responsesased on the same principles and include the samections. For Loviisa NPP, the structure of the EOPs

he actions are somewhat different. The EOPs of Lsa are currently undergoing a wide revision.

.2. Accident scenarios and boundary conditionsWP1)

The first task included analyses of importantube rupture sequences using integral codes. Thebjective was to obtain a basis for the definition oferimental conditions for the ARTIST and HORIZOxperiments. The second task included the defin


Table 1MELCOR calculations of the conditions in the faulted vertical SG during fission product release

SGTR scenario Timing ofinterest (1000 s)

Pressure (bar) Steam temperature (K) Flow through SGbreaks (g/s)

SG Waterlevel

Primary Secondary Primary Secondary Steam Aerosols

Base case (BC) 178–188 30 3.1 500 500 1300 1.18 03-Tube failure (3T) 89–100 18 5.0 475 475 2000 5.16 0Feedwater recovery (FW) 178–180 8 1.1 450 450 600 0.67 0Early FW recovery (EFW) 139–143 33 3.3 500 500 1500 0.05 0

of the experimental boundary conditions based on theresults of the first task. The final test matrices for the ex-perimental studies were determined using the obtainedboundary conditions together with the constraints ofthe experimental facilities and project objectives.

3.2.1. Accident scenarios and boundaryconditions of vertical steam generators

For the vertical type SG, the work was performedmainly by NRG using MELCOR 1.8.3 code. NRGwith the help of PSI extended the existing MELCORmodel of the Beznau NPP. In addition to this SC-DAP/RELAP5 calculations were performed by PSI inorder to simulate a beyond design basis SGTR accidentfor the Beznau NPP.

The most dominant SGTR accident scenario forvertical steam generators was based on informationfrom the Beznau and Borssele PSAs, and on discus-sions between PSI and NRG. The chosen base case(BC) scenario presented a single-tube guillotine typetube failure with a consequentially stuck open safetyrelief valve (SRV). The scenario included the avail-ability of accumulators, of high-pressure emergencycore cooling system (ECCS) until the tank was emptyand of reactor coolant pumps until voiding. Steamdump at the intact SG was performed on basis of startof core heat-up. Recovery of feedwater to the failedSG was assumed after extensive core damage. In thisscenario, high pressure, high steam temperature andlarge masses of fission product were produced. Thers

sec-o of at ultedi re ins ought

More prototypical accident scenarios for determi-nation of the experimental boundary conditions weretwo scenarios with earlier Feedwater recovery. In theFW case, the feedwater is recovered after failure ofthe top part of the core. In the fourth scenario (EFW),the feedwater system is recovered after heat-up of thetop part of the core, which also triggers the perfor-mance of the steam dump. The boundary conditionsbased on these two scenarios are also presented inTable 1.

3.2.2. Accident scenarios and boundaryconditions of horizontal steam generators

The accident scenarios of the horizontal steam gen-erators were mainly calculated by NRI using MEL-COR 1.8.3 code. The calculations were based on PSALevel 2 study performed for the Loviisa NPP. Severalsteam generator tube rupture (SGTR) including mul-tiple tubes rupture (MSGTR) cases were analysed forLoviisa NPP. For Dukovany NPP steam generator col-lector break (SGCB) scenarios were analysed. The dif-ferences between Loviisa NPP and Dukovany NPP inpressuriser and in secondary system valve design andsettings and in operation procedures were taken intoaccount. SGCB scenarios for Loviisa NPP had beencalculated earlier by VTT (Pekkarinen, 1996).

The base case scenario was defined by NRI and For-tum as a double-ended break of the uppermost tube ofthe sheet at the hot primary collector. All safety sys-tems were assumed to work normally. Operator actionsw tingp tucko stemd f cored

anda Theo ave

esulting boundary conditions, presented inTable 1,hould be considered as maximal values.

To study the effect of a larger break in the SG, and ‘worst-case’ scenario (3T) with the assumption

hree-tube failure was analysed. This scenario resn lower pressure in primary side and higher pressuecondary side. The flow of steam and aerosols thrhe break was also higher.

ere taken according to Loviisa emergency operarocedures (EOPs). As a secondary failure, the spen SG safety valve was assumed. Primary syepressurisation procedure started at the onset oamage.

In other scenarios the break location, sizeccident management procedures were varied.verview and the range of the boundary conditions h


Table 2Main results for the horizontal SG accident scenarios

Scenario Average pressure(bar)

Average steamtemperature (K)

Average flow through break (g/s)a

# Descriptionb Primary Secondary Primaryc Secondary Steam Aerosols CsI

LO1 Loviisa SGTR stuck-openSG relief valve

2–8 1.5 473–573 473 150 0.06 0.006

LO1A Like LO1, no primary de-pressurisation

50–95 2 673–1073 573–873 2000 1 0.1

LO2 Like LO1, break locationcold collector

2–8 1.5 453–523 473 150 0.03 0.003

LO3 Like LO1, break locationother loop

2–8 1.5 473–573 473 200 0.04 0.004

LO5 Main steam isolation andfeedwater failure

2–10 1.2 473–623 473 200 0.04 0.004

LOM1 Loviisa MSGTR Like LO1,LO2 tubes

2–10 1.5 453–623 453 250 0.2 0.02

LOM2 Like LO1, LO5 tubes 1.5–9 1.5 473–673 453 150 0.7 0.07DUC1 Dukovany SGCB

stuck-open relief valve toatmosphere

2–4 2–3 473–673 453 1000–4500 6 0.9

DUC2 Stuck-open SG relief valve 2–7 2–6 473–773 473 1000–4000 7 0.6a From the onset of core damage. The average flow is given, in reality, the flow of gases and aerosols varies by about a factor of 10. It is much

higher at the start of fission product release.b SG primary and secondary side water level zero or negligible for all scenarios.c In the hot collector, cold collector for LO2.

been obtained based on these analyses and are shownin Table 2.

The main purpose of the analyses was to obtain typ-ical conditions for integral experiments of horizontalsteam generator. Some general conclusion can also bemade from the analyses. Primary system depressurisa-tion was found to be a very efficient in reducing fissionproduct release to the environment. Upon depressuri-sation, the release was reduced by factor of about 20.The reason for this is that the opening of the pressurerelief valve opens a path for radioactive material to thecontainment instead of releasing it through the breakto the secondary system and to the environment.

3.3. Experimental investigations

3.3.1. Integral studies of vertical SGThe integral tests of vertical steam generators were

conducted at PSI in ARTIST facility, which is a rep-resentative scaled-down model of Beznau referencePWR steam generator. The facility consists of a bundle,shroud, flooding system and aerosol sampling stations.Only the bundle section of ARTIST (including a breakstage, two far-field stages and a U-bend section) was

used in this project. A picture of the facility is presentedin Fig. 1.

The ARTIST test section was directly connected toDRAGON aerosol generation facility. Aerosols wereproduced via fluidised bed aerosol generators (FBG) inconjunction with a venturi injection system. In this pro-gram, prefabricated TiO2 powder was used with a pri-mary particle size of 0.035�m (AMMD). The aerosolmixture was transported to ARTIST test section by car-rier gas composed of steam and non-condensable gas(N2) in desired proportions.

A sophisticated aerosol measurement system wasattached at the inlet and outlet piping to characterisethe aerosol particle size and concentration as well asthe gas flow rates, the gas pressure and gas/water tem-perature. The aerosol characterisation was performedusing state-of-the-art instruments. Two photometersprovided relative aerosol concentration in real-timeat the inlet and outlet. The size distributions weremeasured with Berner low-pressure (BLPI) andAndersen impactors, and the integral concentrationmeasurements were performed with membrane filters.A more detailed description of the facility can be foundin the SGTR project deliverable (Dehbi et al., 2000).


Fig. 1. Photo and a schematic representation of the ARTIST test section.

The actual conditions for the experimental testswere derived from the RELAP5/SCDAP calculations(Guntay et al., 1999) and from report byBakker(2001a), while keeping in mind the practical limits im-posed by ARTIST facility. Five tests comprised the PSIEU-SGTR experiments. The test matrix is shown inTable 3. The first three tests dealt with the aerosol re-tention in the break stage under dry (A01 and A05)and wet (A02) conditions. Test A05 was a repetitionof the test A01. The other two tests addressed accidentmanagement (AM) issues, whereby the SG bundle goesfrom a fully dry state to a fully flooded state. Test A03was performed with a non-condensable (NC)-rich car-rier gas, while test A04 was performed with a steam-rich mixture. An axis-symmetric guillotine break wasused and located 300 mm above the tube sheet in themiddle of the bundle. The aerosol AMMDs at the inletwere 2.25–3.70�m, while at the outlet, the AMMDswere in the range 0.49–0.84�m.

The following conclusions can be drawn from theinvestigations of integral effects in the ARTIST 4 mheight scaled bundle (Table 4):

• With dry bundle and full flow representing the breakstage conditions, there is strong evidence that theTiO2 aerosols (AMMD 2–4�m, 35 nm primary par-ticles) disintegrated into much smaller particles be-

cause of the sonic conditions at the break. Thebreakup promoted particle escape from the sec-ondary and lowered the overall DF, which was typ-ically small, i.e. between 2.5 and 3. Further investi-gation is needed in order to determine the influenceof the type of aerosol on the disintegration process.

• With dry bundle, and small flow reproducing the far-field velocities (test A03), the DF was of the orderof 5, implying better decontamination than with thefull flow. This could be explained by the somewhatlower particle disintegration than witnessed with thelarger flow. The far-field retention implied a DF ofthe order of 1.9 per stage, which, for SG with 9 ormore stages, can translate in overall DF of severalhundreds, when the break is located near the tubesheet.

• With a bundle flooded just above the break and asteam/non-condensable mixture (test A02), the DFwas between 45 and 112 for the full flow and 482for the small flow (typical of far-field). This impliedagain that the far-field stages are more efficient attrapping aerosols than the break stage.

• For the far-field conditions, under a flooded bun-dle and in the presence of steam (test A04), the DFwas roughly of the same order regardless of the wa-ter height, i.e. in the range from 482 to 1081. Alarge fraction of the aerosols was scrubbed at the

A. Auvinen et al. / Nuclear Engineering and Design 235 (2005) 457–472 463Ta

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break level because of strong diffusiophoresis andimpaction of the incoming jet on the water interface.The additional water head beyond the break stagehad only a secondary influence on the magnitude ofdecontamination.

• For the far-field conditions, under a flooded bundleand in the absence of steam (test A03), the DF in-creased exponentially from 124 to 5739, when thewater height in the bundle increased from 1.30 to3.6 m. The aerosol removal rate was roughly con-stant with height, and hence the DF was solely afunction of residence time in the water pool (waterheight).

• When steam was present in the carrier gas underflooded secondary (tests A02 and A04), condensa-tion inside the tube caused aerosol deposition andproduced blockage near the break, with subsequentincrease in the primary pressure. This has implica-tions for real plant conditions, as aerosol depositsinside the broken tube will cause more flow to bediverted to the intact tubes, with corresponding re-duction in the source term to the secondary.

3.3.2. Separate effect studies of vertical SGThe separate effect studies of vertical steam gener-

ators were conducted in the CIEMAT PECA facility,which was properly modified and conditioned for thatpurpose (Peyres et al., 2002). The aim of these stud-ies was to determine aerosol deposition into the breakstage near the break location.

nda dlea tions.A inF p oft le. Itc ide at d int lo-c t thec threet

theo ingt assc heree cas-c n ex-

The PECA facility set-up consisted of gas aerosol injection lines, the vessel with tube mini-bunnd associated instrumentation and sampling staschematic picture of PECA facility is presented

ig. 2. The test mini-bundle was a scaled mock-uhe first stage of the steam generator tube bundonsisted of a squared arrangement housing insotal of 117 tubes plus four supporting rods placehe corners. The mini-bundle allowed two possibleations of the broken tube. One place was just aentre of the structure and the other place wasubes away from the centre.

There was one sampling at the injection line forptical particle counter (OPC) aimed at determin

he aerosol size distribution and quantifying the moncentration at the inlet. Within the vessel atmospight samplings were taken to six filters and twoade impactors, from which the mass concentratio


Table 4Results of tests conducted in the ARTIST facility

Type Test AMMD (�m) Water levelabove tubesheet (m)

DF Phenomena

In Out Disintegration Impaction Diffusiophoresis Rise-zoneinertialremoval

Break stageDry A05 2.25 0.72 0 2.5–2.9 X XWet A02 3.14 0.84 1.30 45.7–112 X X X

Far-fieldAM NC-rich A03 3.70 N/A 0 4.9 X

1.20 124 X X2.30 1251 X X3.60 5739 X X

AM steam-rich A04 2.92 0.49 0 4.6 X1.33 482 X X X2.55 1081 X X X3.80 514 X X X

iting the tube mini-bundle was estimated. In addition tothis, the mass deposited on several selected tubes wascollected and extrapolated to estimate the total aerosolmass in the mini-bundle.

The design of the experimental matrix came fromthe analysis of the prototypical boundary conditionsestimated with MELCOR and SCDAP/RELAP5codes and practical limitations imposed by the facility(Pekkarinen, 1996; Bakker, 2001b). The boundaryconditions were room temperature and inlet pressureof 2.8 bar. The carrier gas was air and the aerosol

product used was prefabricated TiO2 particles. Theexperimental matrix covered two types of break (guil-lotine and fish-mouth), two possible location of thebreak (central and periphery), two possible break ori-entations (facing tube and facing diagonal), and threedifferent inlet gas flow rates (75, 150 and 250 kg/h).With the fish-mouth break type, two different brokenareas were also covered. These were fish-mouth 1Dand fish-mouth 0.5D, whereD denotes the tube equiv-alent diameter. The experimental matrix is shown inTable 5.

Table 5Test matrix for the separate effect experiments of vertical steam generators

Test Break type Break location Break orientation Gas flow rate (kg/h)

Fish Guillotine Central Periphery Facing tube Facing diagonal 75 150 250

1 Xa X X2 Xa X c

3 Xb X X4 Xb X X5 Repetition of test 26 Xb X X7 Xb X X X8 Xb X X X9 Xb X X X

10 X X X X11 X X X X12 X X X X

a 0.5D fish mouth.b 1.0D fish mouth.c Reduce flow rate to a value at which flow velocity is equal to that of test #4.


Fig. 2. Schematic representation of the PECA facility.

The main result of the separate effect studies was arather low global retention in the mini-bundle. Withinthe range of boundary conditions tested, the reten-tion was always below 20% (Peyres et al., 2003). Theamount of retention was much lower than estimated forparticle deposition by inertial impaction. Deposition ofaerosols on the tubes was not uniform along the mini-bundle. The nearest tubes to the break showed depositsforming crusts while the outer tubes showed a thin layerof aerosols. As presented inFig. 3, the highest amountof deposit was found from the mini-bundle with thelowest flow rates 75 and 100 kg/h. Evidently, depositremoval by erosion and resuspension significantly in-fluenced the results with higher flow rates.

The aerodynamic mass median diameter (AMMD)of particles decreased from 6�m at the inlet to approx-imately 3�m at the outlet of the facility. The decreasein the particle size indicated that upon collision withthe surfaces agglomerated TiO2 particles fragmented.This result also indicated that deposit erosion by parti-cle impaction was an important process near the breaklocation.

Important differences between break type and orien-tation were found only at the lowest flow rate (75 kg/h).In case of a guillotine break, the pattern showed asquare symmetry. The tubes up to third neighbours col-lected almost the 70% of the mass retained by the mini-bundle. In fish-mouth tests, the deposition patterns had


Fig. 3. Retention in the mini-bundle vs. inlet gas flow rate.

a triangular shape, where the tubes located far from thebreak point had a low individual deposition. However,far-field tubes taken together represented up to 40% ofthe total retention in the mini-bundle.

3.3.3. Integral studies of horizontal SGIntegral experiments of horizontal steam genera-

tor were conducted in HORIZON facility, which is ascaled-down model of horizontal SG used in VVER-440 (Fig. 4). The objective of the studies was to gather

aph of

data on aerosol behaviour in the primary side of the SGtubes in different flow conditions.

The experiments carried out in the HORIZON facil-ity are shown inTable 6. Aerosols used in the experi-ments were generated by vaporising CsI in the verticalhigh-temperature flow reactor. Depending on experi-ment, a flow of steam or air carried aerosol particlesthrough the reactor into a flow mixer. In the mixeraerosol flow was mixed with superheated main steamflow. Measured particle size (AMMD) ranged from 0.8to 2.7�m and the geometric standard deviation (GSD)of the size distribution from 1.4 to 1.6 at the inlet of theSG (seeTable 7).

The fraction of deposited aerosols was measuredonline by tapered element oscillating microbalance(TEOM) during the experiments. After the experi-ments, the amount of deposited particles was deter-mined by chemical analysis of the filters and SG tubes.In addition, calculations based on thermal–hydraulicresults and on aerosol AMMD were carried out. Theseresults are summarised also inTable 7.

The results of aerosol deposition on the primary sideof the horizontal SG were compared with the valuesobtained from the calculations with the existing de-position models. It appeared that the current modelswere adequate at Reynolds numbers (Re) below 5000,but gave too high deposition velocities atRe above

Fig. 4. A photogr
HORIZON facility.


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Fig. 5. A schematic picture of the PSAERO facility.

70 000 compared to the experimental results. Turbu-lent impaction was considered to be the main deposi-tion mechanism at highRe. However, the effect of re-suspension, which became significant also at highRe,should be added to the models. In all experiments, thedeposited aerosol fraction per unit length had a peakvalue at the tube bend, which confirmed the relativeimportance of particle inertia on deposition. Aerosoldeposition was significantly increased, when the sec-ondary side was flooded with water. Nevertheless, stillmost of the aerosol injected into the tubes was trans-ported as aerosol out of the tubes. Therefore, floodingwas considered to influence mainly the secondary sideretention mechanisms such as pool scrubbing.

3.3.4. Separate effect studies of horizontal SGSeparate effect experiments of horizontal steam

generators were conducted in PSAERO facility. Theseparate effect experiments were designed to com-plement the integral experiments conducted with theHORIZON facility. The objective of the experimentswas to gain mechanistic understanding about aerosolbehaviour in the steam generator tubes.

In PSAERO facility (Fig. 5), the aerosol behaviourwas studied in a straight 3-m long stainless steel tube.The inner diameter of the tube was 13 mm and thelength of the measured section was 2 m. Aerosol de-position and the movement of the deposited materialwas determined by activating the aerosol in a nuclearr tiond itionp ningt ac-t itionw ersf be.

eactor and applying sequentially placed scintillaetectors in online measurements. The final deposrofile was obtained after the experiment by scan

he facility with a similar gamma detector. When inive aerosol was used in an experiment, the deposas determined by sampling with quartz fibre filt

rom the inlet and outlet of the steam generator tu


Table 7Aerosol size distribution in the hot chamber of the HORIZON facility and the amount of deposited aerosols in the tubes as fraction of the amountof injected into the tubes

Experiment Particle size distributionin the hot chamber

Deposited fraction

AMMD ( �m) GSD TEOM Filter Chemical analyses Calculateda

01-A 2.7 1.5 0.30 0.32 0.23 0.2104-A 1.4 1.6 0.20b 0.51 0.03 0.0306-A 1.9 1.6 0.90 0.97 0.19 1.0007-A 1–2 N/A N/A 0.94 0.23 0.94c

08-A 0.8 1.4 0.25 Err.d 0.10 0.2509-A 1.3 1.6 N/A N/A 0.25 N/A

a Calculated with the real particle size assuming density of 4510 kg/m3.

b Point estimate made from 290 to 320 min.c Selected value corresponds to particle size of 0.67�m.d Results in negative value.

In the experiments, polydisperse copper particleswere used as aerosol source. In last experiments, cop-per aerosol was coated with either dry or liquid NaOH.With a scanning electron microscope (SEM), the par-ticles were observed to be separate, nearly sphericaland dense. The particle size distribution was deter-mined with Berner low-pressure impactors (BLPI). Theaerosol mass size distribution was bimodal and it couldbe presented as a sum of two lognormal distributions.The smaller peak, with aerodynamic mass median di-ameter (AMMD) of 0.66�m and logarithm of the ge-ometric standard deviation (lnσg) of 0.65, contained17% of the aerosol mass. The AMMD of the largerpeak was 8.31�m and lnσg was 0.88. Aerosol masssize distribution did not change during the experiments.

The test matrix for the resuspension experimentsis presented inTable 8. In PSAERO experiments, theaerosol was always deposited with a constant gas flowrate. In the first two experiments, deposition and resus-pension phenomena were studied using a high flow rate.From experiment 3 onwards, the flow rate during the

deposition phase was low. After the deposition phasethe gas flow rate was increased stepwise and the de-position profile in the tube was measured online usingradioactive tracer. The effect of material properties onthe deposition–resuspension phenomena was studiedby modifying the surface of the particles in experiment5 and by changing gas composition in experiment 6.

As a result, the mass of particles in the tube was ob-tained as a function of time and location. Spatial reso-lution of the developed online technique was 2 cm andtime resolution was 5 s. From that information, localresuspension rate into a pure gas flow could be cal-culated. Several conclusions could be made from theresults:

• Very little aerosol deposited in experiments con-ducted with a constant high gas flow rate. With flowrates of 100 and 200 l/min particles hitting the sur-face mainly bounced back to the gas stream. Theresult was very similar to that found in HORIZONand PECA experiments. Also, even if the flow rate

Table 8The test matrix for the separate effect studies of horizontal steam generators

Experiment Dep. flowrate (l/min)

Resusp. flowrate (l/min)

Inlet Re Gas inlettemperature (◦C)

Gas Aerosol material Concentration(g/m3)

1 200 200 15500 218 N2 Cu 7.12 100 100 7700 224 N2 Cu 5.63 38.6 40–100 4680–11700 22 N2 Cu 10.44 225 2526 239

55.6 60–80 6980–931060 60–80 4490–598060 60–90 4410–6660

N2 Cu 6.7N2 Cu + NaOH 4.1N2 + H2O Cu + NaOH 3.6


during the deposition phase was low, less than mono-layer of particles remained on the surface, when theflow was increased up to 100 l/min.

• When the flow rate during the deposition phase wasdecreased, the amount of particles deposited on thesurface increased. According to deposition mod-els, the main deposition mechanisms were turbulenteddy impaction and settling. However, the deposi-tion profile in the tube was much smoother than thatestimated with the deposition models. The probablereason for the discrepancy was that the impactionof large particles must have caused erosion. A sig-nificant fraction of the already deposited particleswere knocked of from the surface and subsequentlydeposited further downstream. The total amount ofdeposit was also significantly underestimated by tur-bulent impaction models, which use dimensionlessparticle relaxation time as parameter (Papavergosand Hedley, 1984). It seems that the effect of flowReynolds number should be taken into account inthe modelling of turbulent impaction. Also, the highdensity of particles applied in the experiments mayhave influenced the results more than estimated bythe model.

• The flow rate during the deposition phase had avery significant influence on the strength particlesadhered to the deposit layer. Particles deposited ina higher flow rate were much harder to resuspendthan was the case with a lower flow rate. It is likelythat particles impacting on the surface packed the de-

eenherruc-alsomin-the

• aceben ofnear

de-ber

elyof the

thee the

• In experiment 5, copper particles were coated withdry NaOH, which made the surface of the particlesrough. Increased surface roughness decreased theadhesion force of the particles. Unlike in other ex-periments, significant resuspension took place in ex-periment 5 even during a constant low gas flow rate.Thus resuspension was strongly time-depended. Par-ticles resuspended from the surface also depositedfurther downstream. As a result, the deposition pro-file in the experiment had a wavelike form. Thepeak of the wave moved slowly downstream duringthe experiment. In other experiments, the amountof deposit decreased exponentially as the distancefrom the tube inlet increased. The difference in thedeposition profile was likely due to different parti-cle removal mechanisms. In experiment 5, the mainremoval mechanism was resuspension into the gasstream, whereas in other experiments particles weremainly removed by erosion.

• Results from these experiments were very well com-parable to previous studies on resuspension con-ducted with polydisperse aerosol (Biasi et al., 2001).However, experiments with monodisperse aerosolwithout an exception result in a much higher resus-pension. It is likely that the adherence of polydis-perse aerosol is much better, because particles in thedeposit layer have more contacts to other particlesthan is the case with monodisperse aerosol. A majorproblem in resuspension modelling is that the effectof polydispersity is not taken into account. How-

ery, pa-withtionrse

3

3s

del,A -p besw idents andg im-p s are

posit and increased the number of contacts betwparticles. Therefore, particles depositing with higmomentum would result in a stronger deposit stture. In a higher flow rate, the deposit layer wassubjected to stronger removal forces. Thus theimum force, in which particles could adhere tosurface, increased with the flow rate.In most experiments, deposit removal took plmainly immediately after the flow rate in the tuwas increased. In such occasion, a small fractioparticles was resuspended into pure gas streamthe inlet of the tube. The fraction of removedposit increased further downstream as the numof particles in the gas flow increased. It is thus likthat the resuspended particles caused erosiondeposit layer. In most experiments, erosion wasdominant removal mechanism, probably becausparticles were rather large.

ever, the diameter of a particle is customarily a vimportant parameter in these models. Thereforerameters derived from experiments, conductedmonodisperse particles, should be used with cauin models describing the behaviour of polydispeaerosol.

.4. Model development

.4.1. Aerosol deposition model for the primaryide of the steam generator

In this task, a one-dimensional steady-state moERORESUSLOG (Ludwig, 2002), for aerosol deosition in the primary side of steam generator tuas developed. Based on the most important acccenarios, the turbulent impaction, thermophoresisravitational settling were considered to be mostortant deposition mechanisms. These mechanism


fairly well understood and validated with experimentaldata. In contrary to deposition, the mechanisms respon-sible for resuspension are less well understood, andmore difficult to predict. In order to calculate resuspen-sion, a quasi-static moment balance model byReeksand Hall (2001)was selected for AERORESUSLOGcode. The model seemed to be in reasonable agree-ment with previous experimental results (Biasi et al.,2001).

In the model, the flow field of steam and/or air iscalculated using correlations for heat and mass trans-fer. The mass concentration of the aerosol entering thetube is a lognormal mass-size distribution divided intodifferent size classes. Particle deposition due to ther-mophoresis, turbulent impaction and gravitational set-tling is calculated in each size bin. Thereafter, the re-suspension fraction of deposited particle mass is esti-mated and the mass concentration for each size bin iscorrected with the net deposition.

Integral experiments of horizontal steam gen-erators were calculated with AERORESUSLOG.The calculated results were in good agreement withmeasured values at low (Re= 940) or intermediateReynolds numbers (Re= 3800), where no resuspensionis expected. However, the values calculated at highRe(70 000–140 000) including resuspension were incon-sistent with the experimental results from HORIZONand PSAERO experiments. The reason for differentvalues is most likely that the resuspension model isdeveloped for less than monolayer coverage of parti-c doesn terss positp ticles etterm GTRc

3s

itioni onsw ept’,w e ofo p theg Twom gasi bes.

Second, filtration is considered uniform at any planeperpendicular to incoming gas flow direction.

Under foreseen SGTR conditions, the majordeposition mechanisms in the near-field are turbulentdeposition and inertial impaction. The former domainextends over a Stokes number (Stk) ranging from 0up to 0.1. From this upper bound to higher Stokesnumbers, inertial impaction becomes dominant. Adatabase to develop individual models for turbulenteddy deposition and inertial impaction was set upbased on literature survey (Wong and Johnstone,1953; Ilias and Douglas, 1989). More than a hundredexperimental measurements were compiled andfrom them the following expressions for single tubefiltration efficiencies were derived:

ηtbtST = 4.38× 10−2 + 7.13× 10−2 ln(Stk) (1)

ηimpST = 0.75

1 + 29.31 exp(−3.85Stk0.5)(2)

Using these expressions, total retention efficiency inthe near-field of the tube breach was

ηTB = 1− exp

{− 4dt(dt + s)

4(dt + s)2−πd2t

[1 + (−1)Ntubes+1

×Ntubes∏

i

(Nbins∑

k

y(k)ηST(i, k) − 1

)]}(3)

i chdtotd

od-u re-t blesi un-d I isj tri-b dedo dualm thef oms xer-c

les on a surface rather than for a deposit layer. Itot take into account several important parameuch as erosion, dependence of adhesion on dehysicochemical properties, polydisperse parize distribution and system geometry. Thus, a bodel is needed to describe resuspension in S

onditions.

.4.2. Aerosol deposition model for the secondaryide of the steam generator

In this task, a model to calculate aerosol deposn the near-field of tube breach under dry conditias developed. The model is based on ‘filter conchich means that aerosol flowing through a bundlbstacles is submitted to forces that tend to clean uas by removing particles onto obstacle surfaces.ajor hypotheses lie under this approach. First,

s seen as a viscous fluid flowing transverse to tu

n which Ntubes is the number of tubes over whieposition is considered (i.e., filtration depth),ηST(i, k)

he individual efficiency of a single tubei for particlesf sizek, Nbins the number of particle size classes,y(k)

he mass fraction of particles in size classk, dt the tubeiameter ands the distance between tubes.

The equations were incorporated into a highly mlar FORTRAN-90 code, called ARISG-I (aerosol

ention in steam generators). One of the key varian the assessment of filter efficiency of the tube ble was shown to be the gas velocity. As ARISG-

ust a first step forward in the modelling, major conutions to further develop the model would be neen aspects such as: in-bundle gas velocity, indiviechanisms responsible for aerosol deposition in

ar-field of SG and removal of deposited particles frurfaces. In addition, a more extensive validation eise of the model should be carried out.


3.5. Plant evaluations

3.5.1. Plant evaluations of the vertical steamgenerator

Plant evaluations for the vertical SG were performedbased on the calculations done in WP1 and the experi-ence received during the SGTR project. The purpose ofthese MELCOR calculations was to determine the ef-fect of the developed fission product retention modelson the calculated fission product releases to the envi-ronment, and the impact of accident management mea-sures.

In the cases studied, the impact of the newly de-veloped models on the calculated release was rathersmall. New models predicted higher deposition to theprimary side of the broken SG, which was compensatedby decreased deposition to the secondary side. How-ever, it was shown that the developed models could beimplemented into MELCOR using control functions.The models could be applied also in other scenarios, inwhich retention might be more significant.

According to the calculations, accident managementmeasures influenced not only the deposition of fissionproducts, but also the thermal-hydraulics, the sequenceof the events and thus the fission product behaviour. Itwas observed that early injection of feedwater to thebroken SG, high feedwater flow rate and its high level inthe broken SG decreased the release to the environment.

3.5.2. Plant evaluations of the horizontal steamg

val-u asp re-t tubeb stud-i itht E-S( rios( deo withb

bro-k d tot ction3 h,m d par-

ticles. The effect of deposition near the break locationin the secondary side could be described by a moderatedecontamination factor (DF) of about 1.4–1.7.

An important accident mitigation factor that takesplace, even if no accident management is taken into ac-count, is the aerosol deposition on the secondary sidefar-field tubes and SG shell. This phenomenon was con-firmed for vertical type SG in the ARTIST experiment(Dehbi et al., 2003), and it was included in all plant cal-culations for horizontal SG (Dienstbier and Duspiva,2000; Dienstbier, 2003).

4. Conclusions

The objective of the SGTR project was to provide anexperimental database of aerosol particle retention inSGTR sequences. The work included development ofsimple steady-state models, which were applied in theestimation of fission product retention into the steamgenerator. These models were incorporated into systemlevel code MELCOR. A number of SGTR scenarios inreference PWR and VVER-440 plants were studied inmodel calculations. In these calculations, the effective-ness of different accident management strategies wasassessed in this kind of accidents.

The SGTR project made an important step forwardin resolving uncertainties of various physical models,especially regarding aerosol mechanical resuspension.Work to find a more exact prediction of the effectof aerosol retention in the steam generator duringa jecth on-c effects andf esisa s tou sablea osolp y, ino es,d sions

R

B identl SG

eneratorA two-tier approach was used in the plant e

ations of horizontal SG. The plant analysis werformed first with MELCOR 1.8.3 neglecting the

ention inside the broken tube and on the near-fieldundle secondary side. Particle retention was then

ed in detail using the models developed in WP3 whe help of MELCOR 1.8.5 and modified AERORUSLOG codes (Dienstbier, 2003). Besides of LO1

single tube break) and LOM2 (5-tube break) scenaTable 2), providing no retention on the primary sif the SG tubes, similar scenarios were analysedreak location at half of the SG tube length.

The results indicate that the retention inside theen tube would probably be very small comparehe effect of depressurisation as mentioned in Se.2. Although turbulent deposition velocity was higechanical resuspension removed most deposite

n SGTR accident should be continued. The proighlighted areas where future work should be centrated. These include more focused, separatetudies of aerosol retention in the break stagear-field stages, including the effects of thermophornd aerosol material. Extension of the investigationpper structures (separator and dryer) is also advind would allow a thorough understanding of aerhenomena in the whole steam generator. Lastlrder to model particle removal from the surfacynamic models for deposition and resuspenhould be developed.

eferences

akker, P.J.T., 2001a. MELCOR analysis of Beznau SGTR accscenarios – determination of boundary conditions for verticaexperiments within EU-SGTR Project. SAM-SGTR-D010.


Bakker, P.J.T., 2001b. Steam generator tube ruptures: experimentalboundary conditions. SAM-SGTR-D005.

Bakker, P.J.T., Slootman, M.L.F., 2002. Overview Report onOperational Aspects of SGTR Accidents. SAM-SGTR-D011.

Biasi, L., de los Reyes, A., Reeks, M.W., de Santi, G.F., 2001. Useof a simple model for the interpretation of experimental dataon particle resuspension in turbulent flows. J. Aerosol Sci. 32,1175–1200.

Dehbi, A., Suckow, D., Guntay, S., 2000. Integral tests in a verticalsteam generator bundle: description of the test matrix. SAM-SGTR-D006.

Dehbi, A., Suckow, D., Guntay, S., 2003. Integral tests in a verticalsteam generator. SAM-SGTR-D024.

Dienstbier, J., Duspiva, J., 2000. SGTR scenarios calculation results.SAM-SGTR-D002.

Dienstbier, J., 2003. SGTR scenarios results including new aerosolretention models and the assessment of accident management.SAM-SGTR-D027.

Guntay, S., Birchley, J., Suckow, D., Dehbi, A., 1999. Aerosol trap-ping in a steam generator (ARTIST): an investigation of aerosoland iodine behavior in the secondary side of a steam generator.In: Proceedings of the 27th Water Reactor Safety InformationMeeting, Bethesda, November.

Ilias, S., Douglas, P.L., 1989. Inertial impaction of aerosol particleson cylinders at intermediate and high Reynolds numbers. Chem.Eng. Sci. 44 (1), 81–99.

Jokiniemi, J., Ludwig, L., Herranz, L., 2002. Review of aerosol de-position mechanisms which may be relevant in SGTR conditions.SAM-SGTR-D021.

Ludwig, W., 2002. AERORESUSLOG: model for deposition – resus-pension in the primary side of a steam generator in tube ruptureaccidents. SAM-SGTR-D022.

MacDonald, P.E., Shah, V.N., Ward, L.W., Ellison, P.G., 1996. Steamgenerator tube failures. NUREG/CR-6365, INEL-95/0393.

Papavergos, P.G., Hedley, A.B., 1984. Particle deposition behaviourfrom turbulent flows. Chem. Eng. Res. Des. 62, 275–295.

Pekkarinen, E., 1996. Evaluation of the fission product release to en-vironment using the MELCOR-code in four Loviisa containmentbypass sequences. VTT Energy Research Report ENE4/41/96.

Peyres, V., Polo, J., Herranz, L.E., 2002. PECA facility conditioningand set-up for the SGTR Project. SAM-SGTR-D007.

Peyres, V., Polo, J., Herranz, L.E., 2003. Separate effect studies ofvertical steam generators. SAM-SGTR-D023.

Reeks, M.W., Hall, D., 2001. Kinetic models for particle resuspen-sion in turbulent flows: Theory and measurement. J. Aerosol Sci.32, 1–31.

USNRC, 1990. Severe accident risks: an assessment of five U.S.nuclear power plants, vol. 2. Sandia National Laboratory ReportNUREG-1150.

Wong, J.D., Johnstone, H.F., 1953. Engineering experimental station.University of Illinois, Technical Report number 11.

Wright, A.L. (Ed.), 1994. Primary System Fission Product Releaseand Transport. NUREG/CR-6193.

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