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:
ease in failure probability, the licensee would need to assess the impact that a change in treatment as a result of removal of special treatment requirements might have on the reliability of SSCs. The result of this assessment would be a characterization of the potential impact, which could be qualitative or quantitative. This characterization would need to address the relationship between the elements of treatment being relaxed and their role in maintaining defenses against failure or degradation from known mechanisms. There must be a documented evaluation that provides the development of the quantitative increase in failure probability from the characterization of the impact of the change in treatment. If it is not possible to quantitatively develop a reasonable estimate of the change in reliability, a justified conservative value may be used. The estimate of the change in reliability or the conservative value is used to form the basis for the risk sensitivity study that is performed to show that there is no more than a small net increase in risk associated with implementation of 10 CFR 50.69. The values in NEI 00-04 of two to five are discussed in the document as representing the nominal range between the mean and the 95th percentile values in typical distributions used to characterize current failure rates. The uncertainty in the current failure rates has been developed from the observation of the current population of components (almost all of which are subjected to special treatment requirements) and are developed to characterize the current population. The staff notes that there is no justification provided that selecting the “poor performers” of the current population to represent the failure rates will bound the reliability of components after exemption of 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