Confirmatory Thermal-Hydraulic Analysis to Support Specific Success Criteria in the Standardized Plant Analysis Risk Models—Surry and Peach Bottom (NUREG-1953)

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Publication Information

Manuscript Completed:  May 2011
Date Published: September 2011

Prepared by:
H. Esmaili¹, D. Helton¹, D. Marksberry¹, R. Sherry (NRC retired)¹,
P. Appignani¹, D. Dube², M. Tobin¹,
R. Buell³, T. Koonce³, J. Schroeder³

¹ Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555

² Office of New Reactors
U.S. Nuclear Regulatory Commission
Washington, DC 20555

³ Idaho National Laboratory
P.O. Box 1625
Idaho Falls, ID 83415

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Abstract

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:

• For six SPAR models corresponding to three-loop "high-head" Westinghouse PWRs:

  • Reduction of the adequate venting capability for feed and bleed from two power-operated relief valves (PORVs) to one PORV.

  • Adjustment of the sufficient injection flow during the early stages of a large-break loss-of-coolant accident from two accumulators to one accumulator or one high-head safety injection pump.

• For SPAR models corresponding to BWR Mark Is and Mark IIs:

  • Credit for two control rod drive (CRD) pumps providing adequate core cooling flow following the initial successful operation of the high-pressure coolant injection system or reactor core isolation cooling system.

  • Credit for one CRD pump providing adequate core cooling for injection late in the accident sequence (if not already included).

Some additional changes supported by the MELCOR analysis are not implemented because they are limited by other SPAR modeling assumptions (e.g., timing of core damage relative to battery depletion for station blackout sequences).