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:
pressure equals the vapor pressure corresponding to the temperature of the suppression pool (for BWRs) or sump pool (for PWRs). To reduce the likelihood of a preexisting leak in containment, licensees proposing to use any portion of the CAP that is above the normal operating pressure (hmin) in Figure B-9 or above the accident pressure (hvp) in Figure B-8 should do the following: a. Determine the minimum containment leakage rate sufficient to lose the CAP needed for adequate NPSH margin. b. Propose a method to determine whether the actual containment leakage rate exceeds the leakage rate found in (a) above. For inerted containments, this method could consist of a periodic quantitative measurement of the nitrogen makeup, performed frequently enough to ensure that no unusually large nitrogen makeup occurs. Another method would be to monitor oxygen content. For subatmospheric containments, a similar procedure could be used. c. Propose a limit on the time the plant can operate while the actual containment leakage rate exceeds the leakage rate determined in (a) above. Containment Cooling during an Event in Which CAP Is Used It should be demonstrated that operation of containment sprays and fan coolers will not cause the CAP to be less than that needed to maintain adequate NPSHa. Operator action to control the containment pressure by means of containment sprays or fan coolers is acceptable, if justified. Adequate guidance should be included in the appropriate procedures (emergency, abnormal, and other procedures). B-8 Quantifying Net Positive Suction Head Margin There are several ways to quantify the margin between the expected (realistic, best estimate, nominal) value of NPSH margin and the NPSH margin obtained from licensing calculations. For BWR special events, 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