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:
realistic containment calculations are acceptable. This is consistent with NRC staff guidance in Section B-4 for these events. Realistic calculations imply that no conservative bias is built into the calculations. Conservative assumptions such as the single-failure assumption are not DG-1385, Appendix B, Page B-24 necessary. Input values may be those associated with normal operation, rather than values based on TS LCOs or bounding assumptions (e.g., 100-percent drywell relative humidity). Where a realistic value is not available or cannot be easily defined, a conservative value should be used. For example, the service water temperature may vary over a wide range (depending on the season), so the service water temperature giving the more limiting NPSH margin should be used. For special events, the NPSHr may be used without considering its uncertainty. For DBAs, a conservative (bounding) NPSH margin analysis should be used. Input values should be based on bounding values for significant parameters, and TS LCOs should be used where applicable. NRC staff calculations for the BWR/3 Mark I containment have shown that conservative calculations of NPSH margin fall close to the 95/95 lower tolerance limit of a Monte Carlo calculation of the same problem. This serves to quantify the margin in the conservative calculation. In addition, a realistic calculation should be performed to compare with the conservative calculation. This will also provide a measure of the margin in the conservative calculation. It is also acceptable to perform a Monte Carlo calculation, using the 95/95 lower tolerance limit of available NPSH for the conservative case. For DBA calculations that credit CAP, the NPSHr used should include its uncertainty. B-9 Guidance Summary The NRC guidance for the use of CAP in determining the NPSHa of safety-related pumps, as discussed in Section B-3, is summarized below. B-9.1 For DBAs, analyses involving CAP should use a value of NPSHreff that includes the