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:
e represented in the PRA models as a set of logically linked basic events. Some PRA codes are not well suited to the development and quantification of system level importance measures. One alternative technique uses basic event importance measures (readily calculated by most PRA codes) to identify a set of system functions that are clearly high safety significant. This technique is based on recognition that system function Recovery actions include human actions performed to return a failed system or component to operability. Recovery actions may also include using systems in relatively unusual ways. The procedures for recovery actions usually give only general guidance instead of step-by-step procedures and are not part of the standard training routine. RAW and FV importance measures will always be at least as large as the RAW and FV for basic events whose failure will fail the function. If other importance measures are used with this technique, this property should be validated for the measures used. When basic events are used to characterize the importance of system functions, the relationship between the failure of the basic events and the system functions they support becomes a critical consider- ation. For example, the RAW of a CCF basic event that fails a set of nominally identical pumps provides a reasonable estimate of the margin of safety the proper operation of the pumps is contributing. If the pumps fulfill only one system function, the RAW of the CCF provides a reasonable estimate of that function's contribution to margin of safety. Any system function modeled in the PRA that is supported by one or more basic events that have importance measures above the guideline values should be initially categorized as a candidate high safety-significant system function. 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