Document: NUREG-0800
Document ID: 4e1d86bd-74a5-4678-8bc0-ffce3ba28ea0
Document Type: srp
Title: RISK-INFORMED INSERVICE TESTING
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0428/ML042880272.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.9.7
CFR Part: 
CFR Title: 

Content:
rall impact of the change on plant risk is small as discussed in the next section. Therefore, when categorizing components that are not modeled in the PRA, licensees must take into account the reasons why these components were omitted in the first place. Although PRAs model many of the SSCs involved in performance of plant safety functions, some SSCs are not modeled for various reasons. However, this should not imply that unmodeled components are not important in terms of contributions to plant risk. For example, some components are not modeled because certain initiating events may not be modeled (e.g., low power and shutdown events, or some external events); in other cases, components may not be directly modeled because they are grouped together with events that are modeled (e.g., initiating events, operator recovery events, or within other system or function boundaries); and in some cases, components are screened out from the analysis because of assumed inherent reliability, or failures modes are screened out because of their insignificant contribution to risk (e.g., spurious closure of a valve). The licensee should either provide qualitative arguments that the proposed change to the unmodeled components do not result in an increase on risk, or demonstrate that the components significant to risk are maintained as HSSC. In classifying components not modeled in the PRA as LSSC, the licensee's integrated decision making process should have determined that: * The component does not perform a safety function, or does not perform a support function to a safety function, or does not complement a safety function. * The component does not support operator actions credited in the PRA for either procedural or recovery actions. * The failure of the component will not result in the eventual occurrence of a PRA initiating event. * The component is not a part of a system that acts as a barrier to fission product release during severe accidents. * The failure of the component