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:
row is to be chosen commensurate with the highest frequency initiating event for which failure of the critical safety function would lead to core damage or a large release. Further, in a risk-informed framework, defense-in-depth must be applied to all potential initiating events. Consequently, the defense-in-depth evaluation must include all initiating events credible enough to be postulated in the PRA; not just design basis events. For example, initiating events such as loss of service water cooling system should be included. Further, the estimated plant-specific initiating event frequencies for all the initiating events must be compared to the ranges identified in Figure 6-1 and each plant-specific initiating event placed in the appropriate frequency range. NEI 00-04 does not provide guidance on the use of the proposed defense-in-depth methodology in sufficient detail for the staff to review and endorse this method. For example, it is not clear if the methodology requires that all trains/systems credited in the defense-in-depth analysis (i.e., those considered in the header row) should be considered safety-significant or allow all of them to be LSS. Therefore, as part of a licensee’s submittal requesting to implement 10 CFR 50.69, the licensee should provide the methodology for addressing defense-in-depth, which the staff will review to ensure that it properly reflects the intent of 10 CFR 50.69. 15. Section 6.2 The NRC concludes that the containment and its related systems are important in the preservation of the defense-in-depth philosophy in terms of both large early and large late releases. Therefore, as part of meeting the defense-in-depth principle, a licensee must demonstrate that the function of the containment as a barrier, including fission product retention 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