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

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

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

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

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

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

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

10. Ballooning and rupture occurs near 1000 K Hoop StrainTemperature

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

12. 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

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

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

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

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

17. Ballooning and rupture occurs near 1200 K Hoop StrainTemperature

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

19. 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

20. TMI-2 Core Nodalization

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

22. Rapid Zircaloy oxidation resulted in initial liquefaction and relocation of core metals Fuel temperatures Liquefaction of UO 2 and ZrO 2 Melting of Zr Control rod melt relocation, onset of rapid oxidation

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

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

25. Molten { (U-Zr)-O 2 } pool continued to grow after water injection B-pump Transient Molten pool radius in core

26. Molten (U-Zr)-O 2 relocates into LP after ECCS injection ECCS Injection Melt relocation into LP Temp. of melt in LP Height of debris in LP