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:
on and removal, is not significantly degraded when SSCs that support the functions are determined to be LSS. The concepts used to address defense-in-depth for functions required to prevent core damage may also be useful in addressing issues related to those SSCs that are required to preserve long-term containment integrity. One way to do this would be to show that these SSCs are not relied on to prevent late containment failure during core damage accidents. An alternative method would be to demonstrate that a potential decrease in reliability of low safety- significant SSCs that support the containment function does not have a significant impact on the estimated late containment failure probability. In essence, what the NRC staff expects is a plant- specific understanding of the effects of containment systems on large late releases and the credit given to these systems in maintaining the conditional probability for these releases. A licensee or applicant can qualitatively argue that an SSC is not relied upon to prevent large late containment failure and is thus low safety-significant from this standpoint. However, if an SSC plays a role in supporting the containment function in terms of large late releases and if the licensee wants to categorize these SSCs as LSS (e.g., because of available redundant systems 8 or trains or because its failure is dominated by factors not related to the SSC), then sensitivity studies should be performed to show that the effects on (i.e., change in) the late containment failure probability is small (i.e., less than a 10 percent increase from the base value) and that the factors such as common cause failures or other dependencies are not important. Where a licensee categorizes containment isolation valves or penetrations as LSS, the licensee 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