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PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE ACCIDENTS

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

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PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE ACCIDENTS

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  1. Regional Workshop on Evaluation of Specific Preventative and Mitigative Accident Management Strategies PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE ACCIDENTS Presented Dr. Chris Allison

  2. Outline • Analysis of SAs • Bundle boiloff – influence of SA models • Bundle quench • CORA-13 – PWR – severe oxidation transient during reflood • TMI-2

  3. 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

  4. Influence of SA models starting below 1500 K Fuel rod temperature above midpoint SCDAP predicted temperatures RELAP RELAP predicted temperatures Time (s)

  5. Oxidation heat generation comparable to decay heat Power - Kw Decay heat Oxidation heat generation

  6. Oxidation limited by Zr relocation Maximum bundle temperature - K Maximum temperature Hydrogen production – g/s Hydrogen production

  7. Axial temperature distribution Temperature - K U-Zr-O relocation Dryout Bottom Top

  8. Oxidation front starts above midpoint H2 generation rate – g/s Zr melt relocation

  9. Ballooning and rupture occurs near 1000 K Hoop Strain Temperature

  10. Zr-O-U Relocation to lower portion of bundle Fuel outer radius including frozen crust Temperature

  11. 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

  12. Oxidation heat generation during reflood >> electrical heating Power - Kw Note: Electrical power shutdown prior to quench Oxidation heat generation Quench Decay heat

  13. Oxidation during reflood results in temperature excursion and renewed melting Maximum bundle temperature - K Maximum temperature Hydrogen production – g/s Hydrogen production Quench

  14. Axial temperature distribution Renewed heating in upper bundle due to reflood Temperature - K Bottom Top

  15. Oxidation of liquid U-O-Zr signficant during reflood H2 generation rate – g/s Zr melt relocation

  16. Ballooning and rupture occurs near 1200 K Hoop Strain Temperature

  17. Zr-O-U Relocation to lower portion of bundle U-Zr-O freezing Ballooning Fuel outer radius including frozen crust Temperature

  18. 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

  19. TMI-2 Core Nodalization

  20. Calculated peak core temperatures and pressures for TMI-2 Temperature Pressure Core uncovery B-pump Transient ECCS Injection

  21. Rapid Zircaloy oxidation resulted in initial liquefaction and relocation of core metals Liquefaction of UO2 and ZrO2 Melting of Zr Control rod melt relocation, onset of rapid oxidation Fuel temperatures

  22. B-Pump Transient resulted in sharp increase in oxidation in middle of core Oxidation rate B-pump Transient Peak core temperature

  23. B-Pump transient cooled lower portion of core Axial nodes 3-5 Fuel temperatures

  24. Molten {(U-Zr)-O2} pool continued to grow after water injection B-pump Transient Molten pool radius in core

  25. Molten (U-Zr)-O2 relocates into LP after ECCS injection Temp. of melt in LP Melt relocation into LP Height of debris in LP ECCS Injection

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