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:
ance is valuable for the following reasons: RG 1.175, Page 14 a. When performed with a series of sensitivity evaluations, the use of PRA to determine importance can identify potential risk outliers by identifying IST components that could dominate risk for various plant configurations and operational modes, PRA model assumptions, and data and model uncertainties. b. Importance measure evaluations can provide a useful means to identify improvements to current IST practices during the risk-informed application process. c. System- or functional-level importance results can provide a high-level verification of component-level results and can provide insights into the potential risk significance of RI-IST program components that are not modeled in the PRA. Appendix A of RG 1.174 provides general guidelines for risk categorization of components using importance measures and other information. These general guidelines address acceptable methods for carrying out categorization and some of the limitations of this process. This section gives guidelines specific to the RI-IST application. As used here, risk categorization refers to the process for grouping RI- IST program components into LSSC and HSSC categories. Components are initially categorized into HSSC and LSSC groupings based on threshold values for the importance measures. Depending on whether the PRA is performed using the fault tree linking or event tree linking approach, importance measures can be provided at the component or train level. In either case, the importance measures are applicable to the items taken one at a time; therefore, as discussed in RG 1.174, while a licensee is free to choose the threshold values of importance measures, it will be necessary to demonstrate that the integrated impact of the change meets Principle 4. The next section of this RG discusses one acceptable approach. PRA systematically takes credit for non-ASME OM Code components as providing support, acting as alternatives, and acting