ALLEGRO GAS COOLED REACTOR SEVERE ACCIDENT PROCESSES 

USING MELCOR 2.2Dr. Gábor L. Horváth

EMUG‐2019Brugg‐Windisch, Switzerland 3‐5 April 2019

horvathlg@nubiki.hu

Contents

• ALLEGRO 75 MW reactor• MELCOR model• 10 inch LOCA Severe Accident• Thermal Hydraulics• Radioactivity release mechanisms• Extent of radioactivity release• Conclusions

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Background ‐ Scope• ALLEGRO 75MW is under development in the frame of V4 countries (Cz,SL,HU,PL) 

• NUBIKI Share: Severe accident calculations

• MELCOR selection has been based on:• Capability to calculate with gas only• Experiment post‐test recalculations (HeFus)• Compare to Cathare (steady‐state and DBC)• ALLEGRO DBC and BDBC calculations

• Main goals – study processes in ALLEGRO gas cooled reactors:• Severe accident thermal hydraulics• Fission product transport• Establish accident management procedures

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75 MW ALLEGRO Modelling Scope

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Containment+GVPrimary+DHR

GV–Guard Vessel

L5

Slide 4

L5 nagyon zsúfolt nekem

mi a DHR HXLG; 01.04.2019

ALLEGRO 75 MW – model parts

• CORE+2 loop primary circuit + pony motor• SCRAM• Secondary circuit• DHR heat exchangers + DHR  gas blowers• Nitrogen accumulators• Guard Vessel and an assumed containment

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Allegro 75 MW – Primary + Core model 

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Allegro 75 MW – Severe Accident

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• 10 inch LOCA SA accident with N2 accu and DHR connected but with loss of water in DHR(loss of heat sink)

• Insulated surfaces:• Reactor vessel wall• DHR system downcomer• Guard Vessel wall and cap• Core boundary HS

• Primary circuit disconnected after LOCA onset

Allegro 75 MW – SA accidents10 inch Coldleg LOCA initial conditions

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Accident N2 accum.

PonyMotor+ Sec. Circ.

DHR‐ HX DHR‐blower

DHR waterreserve

LOCA ON No Natural circ. No 0 m3

GV leak  GV init. pressure

0.1 vol%/d 1 bar

Allegro 75 MW – 10inch LOCA SA events

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Events TimeCold leg LOCAd=0.254 m 0.0

SCRAM 0.25 s

DHR – HX valve ON 20 s

Gap release ring 1 288 s

Fuel cladding temperature >1700 K 4218 s

Core Support plate fails by over‐temperature in R1 3.2 d

Core Support plate fails by mass loss in R1 31.9 d

Vessel failure 32dGuard Vessel failureContainment cavity core‐concrete interaction is ON

33d34.5d

Allegro 75 MW – Severe Accident –Very long time until LH failure

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ALLEGRO 75 MW 10 inch Cold leg LOCA Main processes

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ContainmentGravitation: 150x 11.5d

DHR HX– thermophoresis:36x 1000s

GV–Gravitation + thermophoresis12x 2.3d

Allegro 75 MW – BDBA accidentsSevere Accidents – conclusions

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• Main reasons of very delayed LH failure:• Low power of the reactor• Relatively small core and a lot of surrounding metal• Reflector and shielding above the core provide steel to

be heated up stabilizing the core nodes temperature for a long time

ALLEGRO SA: 10” CLeg LOCAwith Loss of HSRadioactivity distribution after 1 day

2019.04.05. 13

ALLEGRO SA: 10” CLeg LOCAwith Loss of HSRadioactivity distribution after 3 days

2019.04.05. 14

Only Xe increase  aft 3 days

ALLEGRO SA: 10” CLeg LOCAwith Loss of HSRadioactivity distribution after 34 days

2019.04.05. 15

Cont: Xe appearsGuard Vessel incr.Xe, Te, Ba, Cs

Allegro 75 MW – Severe Accidents – Vessel failure

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• All aerosols remain deposited in Prim.circ+GV+DHR Hx

• however

• After vessel failurea surge of airborneaerosols (like: Cs)to GV appears

• Reason forsurge is atemperature surge

Allegro 75 MW –MELCOR Experience

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• MELCOR 2.2 managed to calculate ALLEGRO cooled rector SA

• Code often crashed after RPV failure at start of core-concrete interaction

• Typical user error was a small T/H mass with heat source resulting in over 10000 K temperature

• How is the Cs modeled if no water is available in the system?

Allegro 75 MW

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Thank you for your attention

The work has been supported by theHungarian National Research and Development Fond

Under VKSZ_14-1-2015-0021