Loss of essential service water in LWRs (GI-153)

The contribution of essential service water (ESW) system failure to core damage frequency has long been a concern of the NRC. The objective of this study is to assess the safety significance of the loss of ESW systems in LWRs relative to core damage frequency (CDF) and perform a limited value/impact analysis of potential modifications to solve ESW vulnerabilities using a prototypical (pilot) plant. Previous studies indicate that service water systems contribute from < 1% to 65% of the total internal CDF. For the pilot plant analyzed, common ESW vulnerabilities are failure of standby service water pumps to start, backflow through check valves for cross-tied pumps, and failure of normally closed isolation valves in diesel generator cooling loops to open on demand. For the potential modifications evaluated for the pilot plant, the results showed that they could reduce the CDF by as much as 33 percent. However, the dollars per person REM measures resulting from various groups of these modifications significantly exceeded the current criteria of $1000. The results, since they only apply to the prot plant, are not typical of all LWRs. Due to the importance of service water to CDF and the plant specific nature of ESW systems, there could be plants for which there would be cost-effective modifications. Additional analysis would be required to identify them