Document: NRC Regulatory Guide
Document ID: 47b09be1-4bf8-45f9-a099-7fed871c09bd
Document Type: regulatory_guide
Title: Plant-Specific, Risk-Informed Decisionmaking: Inservice Testing (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2114/ML21140A055.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.175
CFR Part: 
CFR Title: 

Content:
, to account for the potential for unknown and unforeseen failure mechanisms or phenomena that, because they are unknown or unforeseen, are not reflected in either the PRA or deterministic engineering analyses. The staff requirements memorandum (SRM)-SECY-98-144, “Staff Requirements—SECY-98-144—White Paper on Risk-Informed and Performance-Based Regulation,” dated March 1, 1999 (Ref. 19), provides additional information on defense-in-depth as an element of the NRC’s safety philosophy. The engineering evaluation should demonstrate whether the implementation of the proposed IST program change is consistent with the defense-in-depth philosophy as described in Section 2.1.1 of RG 1.174. The intent of this key principle of risk-informed decisionmaking with respect to the proposed RI- IST program is to ensure that any impact of the proposed change on defense-in-depth is fully understood and addressed and that consistency with the defense-in-depth philosophy is maintained. The intent is not to prevent changes in the way defense-in-depth is achieved. The licensee should fully understand how the proposed change impacts the plant from both risk and deterministic engineering perspectives. In addition, RG 1.174 provides detailed guidance on how to evaluate the impact of a proposed change on defense-in-depth to determine whether that consistency is achieved. The seven defense-in- depth considerations and four layers of defense that are addressed in Section C.2.1.1 of RG 1.174 should be used to evaluate the impact of a proposed IST change on defense-in-depth to determine whether it maintains consistency with the defense-in-depth philosophy. 2.2.2 Safety Margin Evaluation The maintenance of safety margins is also a very important part of ensuring continued reactor safety and is included as one of the key principles in Regulatory Position C.2 of RG 1.174. Regulatory Position C.2.1.2 of RG 1.174 states “[w]ith sufficient safety margins, (1) the codes and standards or