Document: NRC Regulatory Guide
Document ID: 6f0a99f2-d25a-44e3-b7f2-3286449a9752
Document Type: regulatory_guide
Title: Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident (Rev. 5)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2126/ML21266A185.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.82
CFR Part: 
CFR Title: 

Content:
table to the NRC if the licensee demonstrates that the CAP used in the calculation of NPSH margin is less than or equal to the calculated CAP available at that time in the postulated accident. For the postulated DBA, the calculated CAP includes conservative inputs and assumptions intended to minimize the NPSH margin. For BWR special events (e.g., Appendix R fire, SBO, or ATWS), nominal inputs may be used. This guidance is intended to apply to using CAP to determine the NPSH margin of ECCS and containment heat removal pumps in reactor safety analysis, including the ACRS recommendation to include uncertainty in the NPSH calculations. In the staff requirements memorandum for SECY-11-0014, “Staff Requirements— SECY-11-0014—Use of Containment Accident Pressure in Analyzing Emergency Core Cooling System and Containment Heat Removal System Pump Performance in Postulated Accidents,” dated March 15, 2011 (Ref. B-4), the Commission directed the staff to continue to permit credit for CAP and to include the guidance in Enclosure 1 of SECY-11-0014, dated January 31, 2011 (Ref. B-5). DG-1385, Appendix B, Page B-5 Uncertainty Analysis The current approach to calculating NPSH margin assigns bounding values to the parameters used. These bounding values and assumptions are typically based on historically high or low values or on TS limiting conditions for operation (LCOs). The chosen accident scenario is also limiting. For example, the worst pipe break (giving the most limiting NPSH margin) is assumed for the LOCA, and the worst single failure is assumed. It is also assumed that all these limiting conditions occur simultaneously. For the DBA (the LOCA), the BWR Owners’ Group (BWROG), in its topical report NEDC-33347P-A, Revision 2, “Containment Overpressure Credit for Net Positive Suction Head (NPSH),” issued March 2017 (Ref. B-6), has proposed an alternate method of calculating the NPSH margin, in which the CAP is determined by a Monte Carlo calculation. For some