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:
s 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 the NPSHa equals the NPSHr, with no margin specified. This practice is acceptable for the calculated containment pressure for a LOCA because the calculation is conservative and therefore contains margin. Also, following this guidance, the uncertainty in NPSHr is to be included in the calculation. If the calculated NPSHa, under the assumption that the containment pressure is at its pre-accident value, is less than NPSHreff, then the containment pressure is increased so that NPSHa equals NPSHreff. The amount of containment pressure necessary for NPSHa to equal NPSHreff is the amount of CAP used. The amount of CAP used must be less than the total CAP available at that instant. To determine NPSHa, it is necessary to know the temperature of the pumped water, the pressure above the water free surface, and the head loss in the suction piping from the water source (suppression pool in a BWR or sump pool in a PWR). The CAP, water temperature, and water elevation above the pump suction should be calculated with an NRC-approved method. Calculation of CAP and water temperature involves heat and mass transfer processes within the containment and the tracking of the water, gas, and vapor inventory in the containment. It is also necessary to model the plant equipment, such as heat sinks, and to determine the mass and energy released into the containment by the postulated accident or the special event according to NRC-approved or -accepted methodologies. The containment calculations for NPSH analyses should typically be conservative. That is, all parameters that have a significant effect 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