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:
see will need to address the impact of the proposed change in treatment on a case-by-case basis to ensure that the defense-in-depth principle continues to be satisfied. The NRC believes that the first criteria listed for containment bypass needs to also include mitigation of an interfacing system loss-of-coolant accident as well as the initiation and isolation of these events. 16. Section 7.1 NEI 00-04 states that the safety significance of a system function is determined by the highest RAW or the highest FV of the SSCs in the flow path that are modeled in the PRA. The staff notes that the safety significance of functions derived from the proposed process requires a different definition than the safety significance of individual SSCs derived from the RAW and FV values. The safety-significance of an SSC derived from the RAW and the FV values reflect the increase in risk associated with a failure of that SSC and the fraction of the CDF or LERF to which the failure of the SSC contributes, respectively. An analogous system function safety significance would reflect the increase in risk associated with a failure of that system function and the fraction of risk to which the failure the system function contributes. Most system functions are, however, performed by two or more nominally independent trains and the failure of any one train will not lead to failure of the function. Failure of any individual SSC will, with few exceptions, only fail one train and not the system function. The RAW and the FV associated with the failure of all system trains will be higher, and in most cases much higher, than those associated with individual SSC failures. Consequently, the safety significance of functions derived from the PRA by this process is different and must be clearly differentiated from the safety significance of individual SSCs. An explanation of the difference should be included in the training provided to the IDP. The staff notes that the safety significance of a CCF event