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:
g 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. This figure shows the application of the 100-hour limit between the NPSH margin ratios of 1.2 and 1.6. The NPSH margin ratio is plotted versus time to 100 hours. At the end of this time, in this example, the NPSH margin ratio based on realistic calculations is above the zone of maximum cavitation erosion, while the conservative value is still within the zone. Figure B-5 NPSH margin ratio for RHR pump and zone of maximum erosion DG-1385, Appendix B, Page B-12 B-6 Containment Accident Pressure and Net Positive Suction Head Available To determine the pump NPSH margin, the NPSHa should be calculated from the following equation for the simple flowpath in Figure B-6, which shows a pump taking suction from a closed tank: Figure B-6 Illustration of NPSHa NPSHa = hatm + hstatic − hloss − hvp where— • hatm = the head on the liquid surface resulting from the pressure in the atmosphere above • hstatic = the head resulting from the difference in elevation between the liquid surface and the centerline of the pump suction • hloss = the head loss resulting from fluid friction and fittings in the flowpath to the pump suction flange • hvp = the head equivalent to the vapor pressure at the water temperature If the tank is open to the atmosphere, hatm is the head resulting from atmospheric pressure. Pressurizing the tank increases hatm and therefore increases the pump NPSHa. If the tank is assumed to be the suppression chamber (wetwell) of a BWR or the containment sump of a PWR, the pressure of the atmosphere above the liquid surface is the pressure of the containment atmosphere. During some postulated accidents, the pressure in the containment will increase because of the discharge of steam and flashing hot water into the containment. Because of conditions adverse to NPSH