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:
f special treatment requirements. One mechanism that could lead to large increases in CDF/LERF is extensive, across system common cause failures (CCFs). However, for such extensive CCFs to occur would require that 10 the mechanisms that lead to failure, in the absence of treatment, were sufficiently rapidly developing or not self-revealing, such that there would be few opportunities for early detection and corrective action. Thus, when characterizing the effects of reduced treatment on SSC reliability, the applicant or licensee must consider potential effects of common-cause interaction susceptibility, including cross-system interactions, and potential impacts from known degradation mechanisms. It is expected that those aspects of treatment that are necessary to prevent SSC degradation or failure from known mechanisms to the extent that the results of the sensitivity studies are invalidated will be identified by the licensee and such aspects of treatment will be retained. This will require an understanding of what the degradation mechanisms are and what elements of treatment are sufficient to prevent the degradation. As an example of how this would be implemented, the known existence of certain degradation mechanisms affecting pressure boundary SSC integrity might support retaining the current requirements on inspections or examinations or use of the risk-informed ASME Code Cases, as accepted by the NRC regulatory process. An alternative might be to relax certain elements of treatment, but retain those that were assessed to be the most effective in negating the degradation mechanisms. As another example, changing levels of treatment on several similar components that might be sensitive to CCF potential would require consideration as to whether the planned monitoring and corrective action program, or other aspects of treatment, would be effective in sufficiently minimizing CCF potential such that the sensitivity studies remain bounding. In summary, if this approach