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:
point, the incipient pump cavitation occurs at higher values of NPSH, and the NPSH corresponding to the maximum erosion rate also increases. Hydraulic instabilities may occur in this region. Since the NPSHa depends on the containment pressure, which the operator cannot control (except to limit it using containment sprays and/or fan coolers), the NPSHa will vary during a postulated accident and could fall in the region of the maximum erosion rate for a certain duration. Figure B-4 Erosion zone boundary Pump tests indicate that the zone of maximum erosion rate lies between NPSH margin ratios of 1.2 and 1.6 for pumps operating outside of the zone of suction recirculation (Refs. B-7 and B-8). While pumps with very high suction energy, such as BWR RHR and CS pumps, are subject to cavitation erosion, the time operating in the maximum erosion zone has not been correlated with the degree of damage. There are no technical data available on how long a pump may operate in the maximum erosion zone without failing and on how this cumulative time to failure relates to the pump mission time. A limit of 100 hours is selected for the time permitted in the maximum erosion zone unless the licensee provides a technical basis for an alternate time duration. DG-1385, Appendix B, Page B-11 Figure B-5 shows a sample plot of the NPSH margin ratio (NPSHa/NPSHr) versus time for a BWR/3 with a Mark I containment. Four NPSHa values are calculated. These are based on a conservative analysis, which used conservative inputs, and a realistic analysis, which used nominal inputs, statistical mean values, and statistical minimum values based on Monte Carlo analysis. The value of NPSHreff shown in Figure B-5 is based on engineering judgment of typical numbers and is not applicable to any specific pumping system. Figure B-5 merely illustrates the application of the maximum erosion criterion. Note that the statistical mean values and the “realistic” curve show reasonable agreement in this example.