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:
Potential failure modes considered for systems with this strainer configuration include (1) structural failure of the strainers caused by excessive differential pressure and (2) cavitation of the pump when head loss caused by debris accumulation exceeds the pump NPSH margin. (Note that in a fully submerged configuration, a strainer structural failure, rather than cavitation, may occur, because containment accident pressure may increase the available NPSH, allowing the required flow for the pump to continue to pass through the strainer until a structural failure occurs.) For plants in which ECCS strainers are fully submerged at the time of switchover, the onset of cavitation is determined by comparing the NPSH margin, which is part of the plant’s licensing basis, with the screen head loss calculated in the plant evaluations performed in accordance with Section C.1.3 of this regulatory guide. For this case, therefore, the ECCS strainer failure criterion is assumed to be met when one of the following occurs: • Head loss across the debris bed results in loss of NPSH margin. • Head loss across the debris bed is greater than or equal to the structural limit. Figure A-3a Fully submerged strainer configuration showing solid water from the pump inlet to the containment atmosphere Note that one pump could undergo cavitation while a different pump with a different NPSH margin does not. In certain conditions (see Section C.1.3.1.3), credit may be taken for continued operation under conditions with negative NPSH margin. If justified, short-term operation may be credited to provide an opportunity for recovery action. A-4.2 Partially Submerged Sump Strainers Figure A-3b presents a schematic of a partially submerged strainer. Failure modes for systems with this strainer configuration include (1) pump cavitation, (2) structural failure, and (3) insufficient water entering the strainer because of head loss caused by debris buildup (i.e., flow starvation). The latter failure