Document: NRC Regulatory Guide
Document ID: 2459a562-8984-421c-8058-d096dbb9617c
Document Type: regulatory_guide
Title: Guidelines for Categorizing Structures, Systems, and Components in Nuclear Power Plants According to Their Safety Significance
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0314/ML031430373.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.201
CFR Part: 
CFR Title: 

Content:
ion or mitigation of core damage. To be consistent with the intent of the safety significance categorization process, this first decision block should be broader in scope and include the prevention or mitigation of severe accidents. Further, the logic presented in Figure 5-1 presumes that a negative response to this first decision block means that the follow-on blocks do not need to be addressed. The NRC staff cannot be assured that this screening will eliminate SSCs that are only of low safety significance, especially as currently phrased. Even if a negative response results for this block, the rest of the logic must still be addressed. In essence, NRC would eliminate this initial screening of the system/structure. In Figures 5-2 through 5-7, SSCs having a Risk Achievement Worth (RAW) greater than 2 or a Fussell-Vesely (FV) importance measure greater than 0.005 either on the basis of the base model or sensitivity studies are identified as “candidate safety significant.” Further, throughout this section reference is made that if the external event is a small fraction of the internal events CDF, then safety significance of SSCs considered in the external events PRA can be considered to be LSS from that perspective. The NRC concludes that if a SSC is classified as safety- significant, it cannot be reclassified as LSS by an integral risk consideration. Though the integrated decision-making panel (IDP) may raise a candidate LSS SSC to safety-significant, the IDP cannot lower a safety-significant SSC to LSS. If a SSC is determined to be safety-significant by any of the analyses supporting the risk-informed categorization process, including the appropriate sensitivity studies, then the SSC is safety-significant. Risk Achievement Worth (RAW) is an assessment of the safety significance (i.e., the margin it provides in preventing core damage) of an SSC, whether it be evaluated for a single SSC or a group of SSCs. The RAW value provides the factor that the CDF or LERF