Document: NRC Regulatory Guide
Document ID: a5ee4c78-1135-4bb6-8d54-e974a3402f87
Document Type: regulatory_guide
Title: An Approach for Plant-Specific, Risk-Informed Decisionmaking: Graded Quality Assurance
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1221/ML12216A017.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.176
CFR Part: 
CFR Title: 

Content:
determination of an SSC's safety significance requires determination of the safety significance of all functions the SSC supports. Similarly, determination of the safety significance of support system functions (which should be later pursued in the support system's evaluation) is best performed by determining the safety significance of the function being supported. Licensees may limit their evaluation to the system level and assign all components to the same safety- significance category as the system. This will only reduce the burden on licensees if all the system functions can be categorized as low or medium safety significant. To provide confidence that eventual determination of less than high system safety significance is made with full recognition of each system's contribution to risk, system-level importance should be determined from importance measures developed from the PRA. If the system is not modeled in the PRA, the licensee should determine why the system was not modeled and, guided by this determination, investigate through a traditional engineering review whether any system functional failure will degrade the performance of any human actions or any other systems' high safety-significant functions. A system-level safety significance may be assigned based on the documented results of the review. 2.1.2.1 Quantitative Safety Categorization Insights. Quantitative importance measures from risk studies provide valuable insights about the relative ranking of the safety significance of PRA model elements such as basic events, components, human actions, functions, trains, or systems. At least two quantitative measures of importance are needed, one (such as Fussell-Vesely (FV) or risk reduction worth (RRW)) illustrates the fraction of current risk involving the failure of the model element; the other (such as risk achievement worth (RAW) or Birnbaum) illustrates the margin of safety contributed by the model element's proper operation. Other measures may be used,