Document: NUREG-0800
Document ID: e32f0820-4e33-476e-aa36-4ca8c2c64af0
Document Type: srp
Title: Use of Probabilistic Risk Assessment in Plant-Specific, Risk-Informed Decisionmaking:
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0119/ML011940192.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
conducting an independent verification, reviewers should complete the following steps: * Identify all systems that are relied upon in plant response to an initiating event, whether explicitly modeled in the PRA or not (e.g., room cooling systems, and instrumentation and control systems associated with indications rather than control may not be modeled), and identify the function(s) they perform or support. Determine whether failure of components screened out on the basis that they are elements of "unimportant" systems could affect a system that is relied upon in the plant's response to an initiating event. SRP 19-C5 Reviewers should then verify that at least some elements of each of the important systems identified above are considered "safety significant." If this is not the case, reviewers should ascertain what performaince is allocated to these items in the PRA, and whether the programmatic activities allocated to these elements are commensurate with the given performance level. If a system is identified as being important, but none of its elements is, reviewers should carefully evaluate the licensee's justification. As an example, consider the case of a system that contains many redundant flowpaths. Single-event importance analysis will tend to dismiss the flowpaths one at a time, effectively dismissing thE group as a whole. The focus of the above guidance is that the redundant flowpaths (considered as a subsystem, and recognizing the function they perform), are important and deserve some attention, even though conventional importance measures would not highlight them. However, in the case of redundant systems, the solution need not always be to assign every redundant path to the high-risk contributor category. In this example, especially if the paths are essentially similar, it is arguably necessary to consider common cause failure. Thus, a program that addresses common cause failure potential by monitoring component performance may provide the necessary