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:
re. During some postulated accidents, the pressure in the containment will increase because of the discharge of steam and flashing hot water into the containment. Because of conditions adverse to NPSH margin, such as DG-1385, Appendix B, Page B-13 increased water temperature in the sump pool (PWRs) or the suppression pool (BWRs), or increased head loss at the pump suction screens due to debris blocking the screens, sufficient NPSH margin may not be available in some cases if the CAP is not available. The NPSHa for many PWRs at elevated sump water temperatures (e.g., over 212 degrees Fahrenheit) is determined under the assumption that the pressure at the liquid surface (hatm) equals the vapor pressure (hvp) at the sump water temperature; that is, the following equation applies: NPSHa = hstatic − hloss. This approach ensures that the determination of the NPSHa does not include the partial pressure resulting from the air (or nitrogen) in containment above the liquid surface; the NPSHa value is therefore conservative. However, when the temperature of the sump water is over 212 degrees Fahrenheit, the vapor pressure will be greater than the pressure in containment before the postulated accident, and containment integrity is assumed. As described previously, NPSHr3% is defined as the value of NPSH that results in a 3-percent drop in pump discharge head. The NPSHr3% value is used for two reasons. First, it is relatively easy to determine by testing. Second, most standard low-suction-energy pumps can operate with little or no margin above NPSHr3% without serious effects on their long-term operation. However, the full published pump head is not achieved when the NPSHa equals NPSHr3%. The head is 3 percent less than the fully developed head. It can take from 1.05 to 2.5 times the NPSHr3% value to achieve the 100-percent discharge head. Normal practice in pump operation requires that the NPSHa exceed the NPSHr by some margin. The amount of CAP needed is determined so that