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:
lant’s safety analysis, if applicable; (2) increases the frequency of challenges to the plant resulting from failure of the system; and (3) decreases the reliability or availability of the system to perform its intended functions. For items 2 and 3, the licensee should consider whether any increase in frequency or decrease in dependability results in a significant increase in risk from one type of hazard or scenario. If the risk impact of the proposed licensing basis change is significant, then it is not commensurate with the importance of the system. The ability to accomplish a safety function might be substantially reduced if one of the plant features that provides system redundancy, independence, or diversity is defeated. This adverse impact could occur by the introduction of a new dependency that could potentially defeat the redundancy, independence, or diversity of the affected equipment. Plant changes that introduce new dependencies among systems or functions or that introduce new CCFs (addressed under Factor 4) should not result in a disproportionate increase in risk. Some proposed licensing basis changes allow the plant to be in an operational condition where certain design features are not available to perform their intended functions for some specified period of time. For example, a single train of a multi-train system might be out of service. It is not the intent of this factor of defense-in-depth to preclude such temporary plant configurations. Other controls on temporary plant configurations, such as the Technical Specifications, limit the exposure to risk during such periods. 4. Preserve adequate defense against potential common-cause failures. The proposed licensing basis change should not significantly reduce defenses against CCFs that could defeat the redundancy, independence, and/or diversity of the layers of defense, fission product barriers, and the design, operational, or maintenance aspects of the plant. The evaluation of the proposed