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:
unction. Since it is possible that the system function's RAW and FV measures are much higher than those of any individual basic event, system functions not catego- rized as candidate high should, as a minimum, be further evaluated as discussed below, and the licensee should describe technically how each issue was addressed. The redundancy and reliability of trains within systems that are available to fulfill a critically important system function can have the result that each individual basic event within the system has very low importance measure values or is even truncated out of the results. A system-based evaluation should be performed to determine the impact of the failure of systems that are modeled in the PRA but that have no single failure event (for example, no CCF) and no basic event importance measure above the guideline values. Discrepan- cies in the form of high failure consequence for some systems (automatic depressurization system, for example) but low or no basic event importance measures should be identified and the relevant high safety-significant functions defined and properly categorized as high safety significant. Initiating events are often not modeled as basic events or, if they are, are modeled as single modularized events. Some examples of such initiating events are the loss of instrument air, the loss of main feedwater, the loss of offsite power (through local switchyard faults), the loss of alternating current (AC) or direct current (DC) buses. If components whose failure contributes to these initiating events are modeled in other initiating events (e.g., loss of an air compressor 1.176-9 leading to loss of pneumatic valves following a loss of component cooling), the importance of the basic events will not include the contribution of the failure 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