Document: NUREG-0800
Document ID: 7916b088-fb90-4163-84fe-027bd315bcc5
Document Type: srp
Title: REVIEW OF RISK INFORMATION USED TO SUPPORT PERMANENT PLANT-
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0717/ML071700658.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.2
CFR Part: 
CFR Title: 

Content:
dels are commonly used for events representing the unavailability of a standby component on demand: the standby failure rate model and the constant probability of failure on demand model. The constant probability of failure on demand parameter may be estimated on the basis of an assumed number of demands, implying an average test interval. Use of such a model to investigate the impact of extending test intervals can result in large differences between the unavailabilities of components for which the test intervals differ significantly. Reviewers should be sensitive to this effect, and should ascertain that licensees use appropriate models and parameters for such evaluations. As another example, in considering plant-specific failure data, poorly performing individual components may have been grouped with other components, allowing their poor performance to be averaged over all components of that type. Poor performance may arise because of inherent characteristics of one member of what would otherwise be considered a uniform population, or may arise because components are operating in a more demanding environment. If these components are grouped together with others for which the operating conditions are more favorable, the failure rates used for the poor performers could be artificially lowered. If requirements are relaxed on the basis of the group failure rate, reduced programmatic attention 19.2-52 June 2007 to these poor performers could lead to a greater-than-expected probability of experiencing an inservice failure of one of these components. Reviewers should be aware of such effects, and should ensure that the components are grouped appropriately. When the impact of the change is modeled as a change in the parameter values associated with specific basic events representing modes of unavailability of SSCs, reviewers should focus on whether the change in parameter values is appropriate and reasonable. The licensee is expected to document the rationale behind the