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:
on the containment pressure and water temperature are assumed to be simultaneously at bounding values; these values are typically either TS limits, such as LCOs, or values known to bound the expected value of a parameter. DG-1385, Appendix B, Page B-14 Boiling-Water Reactor Plants In the NRC-approved topical report NEDC-33347P-A/NEDO-33347-A, Revision 2, the BWROG proposed a Monte Carlo method for calculating the lower tolerance limit (e.g., the 95/95 value) of a variable Hww, which is defined as follows: Hww = Pww − Pv ρw where— • Pww = the wetwell pressure above the pool surface • Pv = the water vapor pressure • ρw = density of water The variable Hww involves two of the terms in the equation for the NPSHa obtained from the containment analysis. The other two terms in the calculation of NPSHa are the height of the water surface above the pump suction centerline and the suction piping head loss. The height of the water level above the pump centerline can either be determined in the containment calculation, or set to a conservatively low value. The head loss term in the NPSHa equation is calculated conservatively to bound the expected value. For a LOCA, the head loss term includes the flow resistance due to the accumulation of debris on the suction strainers or screens upstream of the pump suction. BWR licensees proposing to calculate NPSHa using CAP greater than the containment pressure before the postulated accident should follow the procedure in NEDC-33347P-A/NEDO-33347-A. Essentially, this procedure is applied when CAP is needed to ensure NPSHa ≥ NPSHr using conservative assumptions. The flowchart in Figure B-7 illustrates this process. DG-1385, Appendix B, Page B-15 Figure B-7 Flowchart for containment and NPSH analyses DG-1385, Appendix B, Page B-16 CAP head available hatm Time from LOCA ht hvp hmin hatm CAP head available Time from LOCA ht hvp hmin Figure B-8 Illustration of “hatm” and “CAP head available” for Tpmax > Tsat Figure B-9