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:
ure to the initiating event frequency. Thus, the importance of functions whose failure would cause both an initiating event and the partial loss of mitigating function can be severely underesti- mated by surrogate basic event importance measures. PRA's integrated models provide an excellent framework to characterize system and system function importance. One area relevant to GQA that PRA modeling does not usually address, is cross-system dependencies arising from nominally identical components used in different applications throughout the plant. This occurs because cross-system dependencies are typically not modeled (between nominally identical MOVs in different systems, for example) and because the resolution of the PRA models may not be sufficiently detailed (the PRA analyst may not be able to determine whether the circuit breakers in two different systems are identical models, for example). Cross-system dependencies are not modeled in PRAs yet can have a significant impact on risk. Consequently, licensees must develop a monitoring program capable of timely identification of repetitive failures of nominally identical equipment for further investigation. 2.1.2.2 Qualitative Safety Categorization Insights. PRA results are to be used in conjunction with traditional engineering, and the principles associated with defense in depth and safety margins must also be factored into the safety-significance determination. Consequently, the following qualita- tive factors should be applied to the quantitative PRA insights developed in the previous section. The licensee is to be able to describe technically how each issue was evaluated and resolved. The diversity of systems that are able to fulfill critical high level functions (e.g., reactivity control, decay heat .removal) can have the result that each individual system could meet all quantitative guidelines to be categorized in the low safety-significance group. It would be prudent, and the licensee is expected, to designate