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:
del, size, impeller diameter, materials of construction, and pump seal/flush system. • The test is conducted at the same speed as at the plant site. • The test is conducted at the actual predicted NPSHa, since testing at a lower NPSHa can reduce, rather than increase, the cavitation erosion rate in some cases. • The test duration is for the time during which NPSHa is predicted to be less than NPSHreff (LOCA) or NPSHr3% (special event). • The flow rate and discharge head remain above the values necessary to provide adequate core and containment cooling. B-7 Effect of Noncondensable Gas on Pump Mechanical Performance As shown in Figure B-10 (taken from Ref. B-7), the amount of entrained air in a pump increases as the NPSH margin ratio is reduced toward 1.0 and below. The additional entrained air results from the dissolved air coming out of solution as local static pressure drops below the vapor pressure. Centrifugal pumps not specifically designed to transport gas-liquid mixtures can generally accommodate (at inlet pressures near 1 atmosphere) up to approximately 2-percent air volume in the inlet nozzle without appreciable effect (see NUREG/CR-2792). Operation in an airbound condition can cause overheating and failure (seizing of the impeller in the casing of the pump). This damage can occur in 10 minutes or less. Figure B-11 (taken from Ref. B-7) illustrates the impact of air in the pump suction on the pump DG-1385, Appendix B, Page B-19 performance. The figure shows an example of the drop in total pump head as a percentage of the best efficiency point flow rate. There is a large loss in performance when the air fraction is over 2 percent by volume. Larger quantities of entrained air can affect pump mechanical performance, possibly causing complete loss of prime or air binding and mechanical damage. The entrained air may have come from the suction water source or been transported by vortices, or it may be previously dissolved air that has come out of