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PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE
ACCIDENTS
Presented Dr. Chris Allison
Regional Workshop on
Evaluation of Specific Preventative
and Mitigative Accident
Management Strategies
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Outline
• Analysis of SAs– Bundle boiloff – influence of SA models– Bundle quench
• CORA-13 – PWR – severe oxidation transient during reflood
– TMI-2
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Bundle Boiloff
• Two identical bundles– 32 rods in 6X6 array – 0.91 m height– Boildown transient– High decay heat – 58.5 Kw (2.0 Kw/m per rod)
• One bundle modeled using RELAP5 heat structure – 1D heat conduction only
• One bundle modeled using SCDAP fuel rod component – 2D heat conduction, oxidation, ballooning and rupture, material liquefaction
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Axial Power Distribution
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.2 0.4 0.6 0.8 1
Height (m)
Axi
al P
ow
er D
istr
ibu
tio
n
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Influence of SA models starting below 1500 K
RELAPRELAP predicted temperatures
SCDAP predicted temperatures
Fuel rod temperature
above midpoint
Time (s)
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Oxidation heat generation comparable to decay heat
Decay heat
Oxidation heat generation
Power - Kw
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Oxidation limited by Zr relocation
Maximum temperature Hydrogen production – g/s
Maximum bundle temperature - K
Hydrogen production
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Axial temperature distribution
Bottom Top
Temperature - K
U-Zr-O relocation
Dryout
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Oxidation front starts above midpoint
H2 generation rate – g/s
Zr melt relocation
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Ballooning and rupture occurs near 1000 K
Hoop Strain Temperature
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Zr-O-U Relocation to lower portion of bundle
Fuel outer radius including frozen crust
Temperature
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CORA-13 PWR Quench
• Electrically heated PWR bundle– 25 rods (16 fuel rods, 7
heated fuel rod simulators, 2 Ag-In-Cd control rods)
– 1.00 m heated length – Constant steam/argon flow
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Oxidation heat generation during reflood >> electrical heating
Decay heat
Oxidation heat generation
Power - Kw
Quench
Note: Electrical power shutdown prior
to quench
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Oxidation during reflood results in temperature excursion and renewed melting
Maximum temperature Hydrogen production – g/s
Maximum bundle temperature - K
Hydrogen production
Quench
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Axial temperature distribution
Bottom Top
Temperature - KRenewed heating in upper bundle due to reflood
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Oxidation of liquid U-O-Zr signficant during reflood
H2 generation rate – g/s
Zr melt relocation
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Ballooning and rupture occurs near 1200 K
Hoop Strain Temperature
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Zr-O-U Relocation to lower portion of bundle
Fuel outer radius including frozen crust
Temperature
Ballooning
U-Zr-O freezing
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TMI-2
• The TMI-2 problem is described in the SCDAP/RELAP5/MOD3.2 reference manual (Volume V)– General description (Section 5.5)
– Input model description (Appendix A.11)
• TMI-2 sample problem on CD includes– Restart plot file– Sample input file (restarting after B-pump transient
and formation of initial molten pool)– Sample plot input file
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TMI-2 Core Nodalization
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Calculated peak core temperatures and pressures for TMI-2
B-pump Transient
Core uncovery
ECCS Injection
Temperature
Pressure
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Rapid Zircaloy oxidation resulted in initial liquefaction and relocation of core metals
Fuel temperatures
Liquefaction of UO2 and ZrO2
Melting of Zr
Control rod melt relocation, onset of rapid oxidation
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B-Pump Transient resulted in sharp increase in oxidation in middle of core
Peak core temperature
Oxidation rate
B-pump Transient
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B-Pump transient cooled lower portion of core
Fuel temperatures Axial nodes 3-5
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Molten {(U-Zr)-O2} pool continued to grow after water injection
B-pump Transient
Molten pool radius in core
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Molten (U-Zr)-O2 relocates into LP after ECCS injection
ECCS Injection
Melt relocation into LP
Temp. of melt in LP
Height of debris in LP