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

Influence of SA models starting below 1500 K RELAP RELAP 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 StrainTemperature

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 - K Renewed 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 StrainTemperature

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 UO 2 and ZrO 2 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)-O 2 } pool continued to grow after water injection B-pump Transient Molten pool radius in core

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