practical examples of the analysis of severe accidents
DESCRIPTION
Regional Workshop on Evaluation of Specific Preventative and Mitigative Accident Management Strategies. PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE ACCIDENTS. Presented Dr. Chris Allison. Outline. Analysis of SAs Bundle boiloff – influence of SA models Bundle quench - PowerPoint PPT PresentationTRANSCRIPT
PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE
ACCIDENTS
Presented Dr. Chris Allison
Regional Workshop on
Evaluation of Specific Preventative
and Mitigative Accident
Management Strategies
Outline
• Analysis of SAs– Bundle boiloff – influence of SA models– Bundle quench
• CORA-13 – PWR – severe oxidation transient during reflood
– TMI-2
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
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
Influence of SA models starting below 1500 K
RELAPRELAP predicted temperatures
SCDAP predicted temperatures
Fuel rod temperature
above midpoint
Time (s)
Oxidation heat generation comparable to decay heat
Decay heat
Oxidation heat generation
Power - Kw
Oxidation limited by Zr relocation
Maximum temperature Hydrogen production – g/s
Maximum bundle temperature - K
Hydrogen production
Axial temperature distribution
Bottom Top
Temperature - K
U-Zr-O relocation
Dryout
Oxidation front starts above midpoint
H2 generation rate – g/s
Zr melt relocation
Ballooning and rupture occurs near 1000 K
Hoop Strain Temperature
Zr-O-U Relocation to lower portion of bundle
Fuel outer radius including frozen crust
Temperature
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
Oxidation heat generation during reflood >> electrical heating
Decay heat
Oxidation heat generation
Power - Kw
Quench
Note: Electrical power shutdown prior
to quench
Oxidation during reflood results in temperature excursion and renewed melting
Maximum temperature Hydrogen production – g/s
Maximum bundle temperature - K
Hydrogen production
Quench
Axial temperature distribution
Bottom Top
Temperature - KRenewed heating in upper bundle due to reflood
Oxidation of liquid U-O-Zr signficant during reflood
H2 generation rate – g/s
Zr melt relocation
Ballooning and rupture occurs near 1200 K
Hoop Strain Temperature
Zr-O-U Relocation to lower portion of bundle
Fuel outer radius including frozen crust
Temperature
Ballooning
U-Zr-O freezing
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
TMI-2 Core Nodalization
Calculated peak core temperatures and pressures for TMI-2
B-pump Transient
Core uncovery
ECCS Injection
Temperature
Pressure
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
B-Pump Transient resulted in sharp increase in oxidation in middle of core
Peak core temperature
Oxidation rate
B-pump Transient
B-Pump transient cooled lower portion of core
Fuel temperatures Axial nodes 3-5
Molten {(U-Zr)-O2} pool continued to grow after water injection
B-pump Transient
Molten pool radius in core
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