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:
-LOCA subcriticality and shutdown margins. Typically, these calculations are performed assuming minimum boron concentration and maximum dilution sources. Similar considerations should also be included in the calculation of time for hot-leg switchover, which assumes maximum boron concentration and a minimum of dilution sources. The evaluation of debris transport to the sump screen should account for the time to switch over to sump recirculation and the operation of containment spray. 2.1.1 The ECCS strainers should be located on the lowest elevation in the containment, exclusive of the reactor vessel cavity and the normal drainage sump, to maximize the pool depth relative to the strainers. Recirculation strainers should protect the pump inlets for which they supply water. A curb may be provided upstream of the strainers to prevent high-density debris from being swept along the floor into the sump strainer. The height of the curb should be appropriate for the pool flow velocities and plant debris types, because flows of sufficiently high velocity can carry debris over a curb. Estimation of pool flow velocities should include both the pool fill (as applicable) and the recirculation phases of the event. Licensees should also consider that turbulence in the pool may keep some debris in suspension that would otherwise settle. Experiments documented in NUREG/CR-6772 and NUREG/CR-6916 demonstrated that at typical containment pool velocities, some types of settled debris will slide or tumble across the containment pool floor to the suction strainer. The ECCS strainers should include access openings and other design features, as required, to facilitate inspection of the strainer structures, vortex suppressors, and pump suction piping inlets. Where consistent with overall design and functionality, the top of the ECCS strainer structures should be a solid cover plate that is designed to be fully submerged after a LOCA and completion of the ECCS injection phase. The cover plate