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CONTENTS
• Containment failure modes• Induced RCS ruptures• Overview of containment challenges• Hydrogen detonations• Direct containment heating• Long term containment overpressure• Cavity issues in VVER-440/213• Phenomenology related to in-vessel retention
CONTAINMENT FAILURE MODES
• Experience with structural analysis suggestscontainment failure pressures at ~ 2x designoverpressure
• For VVER 440/213 critical locations appear to be:• Bubbler tower corners, SG room wall/floor junction• hatches, mechanical and electric penetrations
• In some severe accident sequences, containment maybe bypassed– E.g., SGTR, V sequence
INDUCED RCS RUPTURE (1)
• High temperature and pressure in RCS during coremelt
• Flow of hot gases:– Core -> upper plenum -> pressuriser surge line -> Pressuriser
Relief Valve– Core -> upper plenum -> loop seal– In VVER loop seal is between hot nozzle and
• Heating & stressing of hot leg, surge line and SGtubes
• Correlations of creep rupture data have beendeveloped, e.g., Larson-Miller, Manford-Hanson– Useful for probabilistic analysis
INDUCED RCS RUPTURE (2)
• Hot leg or the pressuriser surgeline fails– Usually expected to be beneficial for accident
progression• Causes RCS depressurisation, avoids DCH• Not beneficial if low pressure failure of RCS is worst case
– MAAP analyses suggest this is a credible failuremode in VVER
• Induced steam-generator-tube rupture– Detrimental for accident progression (radionuclide
release)– Unlikely in VVER, due to loop seal
SEVERE ACCIDENT PHENOMENA INCONTAINMENT
HYDROGEN PRODUCTION & COMBUSTION IN VVER-440/213
• Key design feature:– bubbler condenser air traps
• code simulations suggest that during the hydrogenproduction phase very little gas flows into air traps
• air traps account for approx 45% of confinement volume
• Combustible mixtures can arise during in-vessel phase
• Burn pressures can exceed median failure pressure
• Detonable mixtures may arise in someconfinement regions in later accident phases
DETONATIONS (1)
• Direct detonation generally not credible– high [H2] > 18 vol % + high energy ignition source required
• Deflagration to detonation transition (DDT) maybecredible but is geometry dependent– “open” geometries less susceptible– Flame front accelerating geometries (e.g., long confined flow
paths with presence of obstacles) more susceptible• Sherman-Berman classification model has been used
in some NPP level 2 PSAs– Ref: Nucl Tech, 81(1988)63-77– Qualitative method:
• Assess geometry (scale, obstacle, degree of confinement etc)• Assess mixture• Result class is function of geometry and mixture class
Measured and calculated detonation cell width of H2-air-steammixtures for 100ºC and constant air density (41.6 mole/m3)
DETONATIONS (2)
DETONATIONS (3)
45555
34455
23345
12345
11234
54321Geometric Classes
12345
Mixture Classes
Dependence of result class on mixture & geometric classes
(from Sherman and Berman)
DETONATIONS (4)
>0.9
0.7
0.5
10-2 – 10-1
10-3 – 10-4
DDT is highly likely
DDT is likely
DDT may occur
DDT is possible but unlikely
DDT is highly unlikely to impossible
Result Class 1
Result Class 2
Result Class 3
Result Class 4
Result Class 5
SZBPSA
DefinitionClasses
Mapping to probability used in Sizewell “B” PSASource: M. L. Ang, “IAEA Workshop on level 2 PSA”, Beijing, November 2001
DETONATIONS (5)
• 7λ criterion (applied to EPRassessment)
R = D / ( 7λ )
R = DDT index, D = characteristic dimensionλ = detolation cell size
> 1 DDT possible<1 DDT not possible
Correlation between Mixture Detonation Cell Size, Geometrical Size and Combustion ModeExperiments from RUT Facility[SMiRT 14, 1997]
Experimental data on turbulent jet initiationand DDT in confined volumes
DIRECT CONTAINMENT HEATING (1)
• Early containment failure mechanism
• High pressure melt ejection (HPME) from RPV– rapid, energetic discharge and fragmentation of melt debris
• Fine particles (~mms) entrained and transported toupper containment
• Large surface area => rapid heat transfer + meltoxidation + hydrogen combustion– pressure spike may challenge containment integrity
DIRECT CONTAINMENT HEATING (2)
• Limiting factors
– De-entrainment by structures and flowpaths• impingement of particles on the many structures present
in the various pathways• highly tortuous flowpaths promote the separation and
impingement of debris
– Limited fine particulate generation observed inexperiments
– Efficiency of heat generation + transfer < 100%
LONG TERM CONTAINMENT OVERPRESSURE (1)
• Two sources of mass addition to containmentatmosphere– Vapour generated during cooling of debris
• Net accumulation in absence of heat removal fromcontainment atmosphere (e.g. no fan coolers, sprays, etc)
• Debris cooled ex-vessel scenario– Non-condensible gases generated during MCCI
• Dependent on concrete composition• Debris not cooled ex-vessel scenerio
– Long term increase in containment pressure &failure
LONG TERM CONTAINMENT OVERPRESSURE (2)
VVER-440/213 CAVITY PHENOMENA (1)
• Cavity design features:– cavity door– small volume, small vent area– small flow area annular gap– dry design
• water for recirculation collects in SG boxes (large volume)
VVER-440/213 CAVITY PHENOMENA (2)
• Code calculations suggest significantpressure rise at vessel breach
• Structure of most concern is cavity door– failed door leads to outside confinement
• DCH looks unlikely
IN-VESSEL RETENTION PHENOMENOLOGY (1)
• Studies suggest that in-vessel retentionstrategy may be viable for VVER & AP600– low power density– no lower head penetrations
• In-vessel retention AM strategy accepted bySTUK for Loviisa
IN-VESSEL RETENTION PHENOMENOLOGY (2)
• NEA/CSNI/R(98)18, “In-vessel core debrisretention and coolability”, Feb 1999(workshop proceedings, 3-6 March, 1998).– Presentation “In-vessel retention as a severe
accident management strategy”, T.G. Theofanus.– Two threats identified: in-vessel steam
explosion/impulse loads thermal loads– Analysis uses loading - resistance concept
IN-VESSEL RETENTION PHENOMENOLOGY (3)
• Failure criteria for thermal loads based onCHF
• Research on CHF for downward-facing,inclined surfaces– CHF is lower for completely horizontal surface– experiments ULPU-2000 (for AP600), SULTAN in
Grenoble
IN-VESSEL RETENTION PHENOMENOLOGY (4)
• Steam “lens” forms, and eventually slipsaway (on curved surface)
• timing influenced somewhat by presence of saturated vs.sub-cooled liquid
• micro-film of liquid maintains heat transferuntil lens slips away
• uncertainty in thickness of micro-film & hence time• If time to dry-out micro-film < time to vent
steam, boiling crisis occurs & vessel heats up• CHF ~ 350 - 550kW/m2• higher on inclined surface
IN-VESSEL RETENTION PHENOMENOLOGY (5)
• relations developed which suggest upwardsheat flux are about 1.5 x downward– average heat flux to wall can be estimated
• variation of heat flux with position in lowerhead also assessed– point of most concern top of debris/wall contact
• concern that a thin metal layer on top ofdebris could collect & focus (conduction towall) heat– key uncertainties: layer thickness & emissivity
