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:
ctions”, it is stated “SSCs which have high failure probabilities (usually indicative of screening values) and meet the screening criteria solely on the basis of Fussell-Vesely importance may have been identified as candidate safety significant.” There is no action associated with this statement. The NRC finds that an acceptable approach to dealing with the issue of SSCs categorized as safety-significant solely 11 on the basis of an artificially high failure probability is to first revise the model, if appropriate, to use the proper value and then to recategorize the SSC. Only through a thorough recategorization effort, which would involve going through the entire process and considerations at the same level of rigor and depth as the original categorization, can a SSC be recategorized lower than its initial categorization. 20. Section 9.2 Under the “Review Defense-In-Depth Implications” subsection, the NRC staff does not agree that low safety significance can be assigned if any one of the criteria listed is true. For the IDP qualitative evaluation to determine the impact of relaxing requirements on SSC reliability and performance, historical data must show that the failure mode is unlikely to occur and either the failure mode can be detected in a timely fashion or there is condition monitoring that provides a leading indicator. Further, the NRC staff interprets this evaluation and criteria to be applicable to both subsections, “Review of Risk Information” and “Review Defense-In-Depth Implications.” In addition to recommending staggered testing, inspection and/or calibration of equipment as strategies for reducing the potential for common cause failures and/or detection of failures, the licensee could take the strategy of not reducing treatment for those SSCs with the potential for common cause failures. The IDP may want to use the following criteria to check whether the SSC is categorized appropriately, and for SSCs not explicitly modeled, by