Document: NRC Regulatory Guide
Document ID: 82659041-98b0-4721-b25d-c4fb2ea394d0
Document Type: regulatory_guide
Title: An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1635/ML16358A153.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.174
CFR Part: 
CFR Title: 

Content:
prevent changes in the way defense-in-depth is achieved. The licensee should fully understand how the proposed licensing basis change impacts plant design and operation from both risk and traditional engineering perspectives. Section C.2.1.1.1 provides a brief background on the defense-in-depth philosophy. Section C.2.1.1.2 provides a discussion of seven factors that should be used to evaluate the impact of the proposed licensing basis change on defense-in-depth. Section C.2.1.1.3 provides guidance on a process for evaluating the seven defense-in-depth evaluation factors, and Section C.2.1.1.4 provides guidance on the integrated evaluation that should be conducted to demonstrate the application of this guidance. Background Defense-in-depth is an element of the NRC’s safety philosophy that employs successive compensatory measures to prevent accidents or mitigate damage if a malfunction, accident, or naturally caused event occurs at a nuclear facility. The defense-in-depth philosophy has traditionally been applied in plant design and operation to provide multiple means to accomplish safety functions and prevent the release of radioactive material. It has been and continues to be an effective way to account for uncertainties in equipment and human performance and, in particular, 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 traditional engineering analyses. The SRM on SECY- 98-144, “White Paper on Risk-Informed and Performance-Based Regulation” (Ref. 27), provides additional information on defense-in-depth as an element of the NRC’s safety philosophy. In addition, some flexibility can be gained in the operations and maintenance of the nuclear plant that leverages the implementation of the defense-in-depth philosophy in the design of the plant. For example, testing and maintenance of SSCs or corrective action to restore an engineered