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:
cted 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-5 Cavitation Erosion and the Use of Containment Accident Pressure One of the adverse effects of insufficient NPSH margin is cavitation that results in erosion (pitting) of impeller blade surfaces and possibly of other parts of the pump, due to the implosion caused by condensation of vapor bubbles near a solid surface. Visual studies, acoustical measurements, and field experience show that the maximum cavitation erosion rate occurs at an NPSHa value between the NPSHr3% value (NPSH margin ratio = 1.0) and the point of cavitation inception (NPSH margin ratio of 4.0 or higher). The exact value will depend on the pump, the amount of air dissolved in the water, and the point of operation on the pump curve with respect to the best efficiency point. Figure B-4, a plot of NPSH versus pump flow rate, is a qualitative representation of the various operating regions with respect to cavitation erosion. The figure shows three curves. The top curve is the NPSH corresponding to incipient cavitation (NPSHi). The middle curve is the NPSH at which the DG-1385, Appendix B, Page B-10 maximum erosion rate occurs (NPSHd). The bottom curve is NPSHr3%. It is noted that the maximum erosion curve lies above the NPSH3% curve. The NPSHa at which the maximum erosion rate occurs can be two to four times NPSHr3%. The NPSH curve of incipient cavitation and the NPSH curve of maximum cavitation erosion have similar shapes. At the bottom point on these curves, the impeller pressure distribution is most favorable, and incipient cavitation and cavitation erosion are minimum. As the flow rate decreases from the bottom 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