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:
et Positive Suction Head (NPSH),” issued March 2017 (Ref. B-6), has proposed an alternate method of calculating the NPSH margin, in which the CAP is determined by a Monte Carlo calculation. For some parameters, input values are sampled from statistical distributions; for others, conservative (bounding) values are used. An acceptance criterion of a 95-percent probability at a 95-percent confidence level (95/95) is used for the Monte Carlo pressure calculation. However, since conservative values are used for other inputs to the NPSHa calculation, the tolerance limit on NPSHa is greater than the 95/95 value. For the special events, realistic input values may be used. Licensees should attempt to quantify uncertainty in the calculation of NPSH margin. Where this is not possible, the NRC staff recommends using bounding values of inputs. B-4 Use of Containment Accident Pressure in Determining the Net Positive Suction Head Margin of Emergency Core Cooling System and Containment Heat Removal Pumps “NPSH Required” and “NPSH Required Effective” NPSHr is a property of the pump. In addition to the pump design, NPSHr varies with the pump flow rate and the temperature of the pumped water. NPSHr corresponds to an acceptable level of pump cavitation: that is, a level that allows the pump to perform its safety function for the necessary period of time. This period includes not only the duration of the accident, when the NPSH margin may be limited, but also any time after recovery from the accident during which pump operation is needed to maintain the reactor or containment in a stable, cool condition; during this time, the NPSH margin may be much greater. The additional time after recovery is usually taken to be 30 days. It should be applied to both the DBA and special events. The staff proposes that the NPSH margin be calculated as NPSHa − NPSHreff, where NPSHreff is the NPSHr3% value with uncertainties included. This calculated NPSH margin should be greater than or