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:
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 solution. The configuration of the sump pool (PWRs) or suppression pool (BWRs) should eliminate consideration of entrained air (for instance, due to air entrained by containment sprays) and vortices, since any air bubbles will rise to the free surface of the pool, and steps are taken in sump design to eliminate vortices. In addition, the data developed for NUREG-0897, Revision 1, “Containment Emergency Sump Performance: Technical Findings Related to Unresolved Safety Issue A-43,” issued October 1985 (Ref. B-11), show that vortices decay to negligible levels within 14 pipe diameters; thus, vortices created in a pool would not reach the pump suction through the pump upstream piping. DG-1385, Appendix B, Page B-20 Figure B-10 Impact of cavitation on entrained air DG-1385, Appendix B, Page B-21 Figure B-11 Illustration of pump total head performance with entrained air DG-1385, Appendix B, Page B-22 Another concern in operating a pump with NPSHa close to NPSHr3% is the possibility that water vapor or entrained air could damage the mechanical shaft seal faces within the pump, which could fail very quickly if the seal faces run dry. Excessive entrained air tends to accumulate near the shaft, where the mechanical seal(s) is(are) housed. Therefore, to protect the mechanical seal faces from the excess entrained air expected when NPSHa is close to NPSHr3%, dual mechanical seals with an external cold water flush system (or equivalent) should be provided. Pump Flow Rate The flow rate chosen for the NPSHa analysis should be greater than or equal to the flow rate assumed in the safety analyses that demonstrate adequate core and containment cooling. This ensures that the safety analyses and the NPSH analysis are consistent. If the CAP used is determined assuming that NPSHa equals