Document: NRC Regulatory Guide
Document ID: c7a40fcc-fc9d-4eb2-ad86-f9f5b0f04c82
Document Type: regulatory_guide
Title: Plant-Specific, Risk-Informed Decisionmaking:  Technical Specifications (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1920/ML19206A489.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.177
CFR Part: 
CFR Title: 

Content:
occurred in the standby period. The component failure rate, λ, represents these failures in the formulation of component unavailability. The test-limited risk is normally estimated by assuming that a surveillance test of a component detects the failures, and that after the test, the component’s unavailability resets to zero, or “false” in the Boolean expression. A few component failures, depending on a component’s design and the test performed, may not be detected by a routine surveillance test. Usually, their contribution to risk is considered negligible. b. Regular surveillance testing of a component, as performed for safety system components, is considered to influence its performance. Generally, for most components, the increase of a surveillance interval beyond a certain value may reduce the component’s performance (i.e., increase the failure rate). Experience data are not available to assess the SF values beyond which the component failure rate, λ, increases. In a risk-informed evaluation of surveillance requirements, if the failure rate is assumed to remain the same (i.e., unaffected by a change in the SF), this assumption implies that the SFs are not being changed beyond the value at which λ may be affected. Care should be taken not to reduce the SFs beyond such values using risk-informed analyses only. c. The timing of surveillance tests for redundant components relative to each other (i.e., the test strategy used) impacts the risk measures calculated. Staggered or sequential test strategies are commonly used. The risk impacts of adopting different test strategies (e.g., sequential versus staggered) should be evaluated to determine whether there is an impact on the evaluation of the change being considered (Ref. 23). d. Notwithstanding the beneficial aspects of testing to detect failures that occur in a standby period, several adverse effects may be associated with the test that should be considered in the SF evaluation, including downtime to