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In a limited number of cases, thermal-hydraulic success criteria from the suite of standardized plant analysis risk (SPAR) models have apparent inconsistencies when compared to counterpart licensee probabilistic risk assessments (PRAs), other relevant SPAR models (i.e., models for similar plants), or relevant engineering studies
These inconsistencies are a natural outcome of the SPAR development process, and often reflect the apparent inconsistencies seen across licensee PRAs for similar plants. Even so, the U.S. Nuclear Regulatory Commission (NRC) staff wants to strengthen the technical basis for the SPAR models by performing targeted additional engineering analysis. The identified success criteria are for both pressurized-water reactors (PWRs) and boiling-water reactors (BWRs). This report describes MELCOR analyses performed to augment the technical basis for supporting or modifying these success criteria. The success criteria contained herein are intended to be confirmatory in nature, and while suitable for their intended use in supporting the SPAR models they are not intended to be used by licensees for risk-informed licensing submittals. This report first provides a basis for using a core damage surrogate of 2,200 degrees Fahrenheit (1,204 degrees Celsius) peak cladding temperature. Following this discussion are descriptions of the major plant characteristics for the two plants used for this analysis (Surry Power Station and Peach Bottom Atomic Power Station) and the MELCOR models used to represent these plants. Finally, the report presents the results of many MELCOR calculations and compares these results to the corresponding sequences and success criteria in the SPAR models for Surry and Peach Bottom. The results provide additional timing information for many sequences, confirm many of the existing SPAR model modeling assumptions, and support a few specific changes. Specific changes that have been made to the SPAR models as a result of these analyses are:
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