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:
is expressed in units per time period, such as per hour. Estimating RD requires the standby stress failure rate of the component and the proposed SF. The failure probability of a component consists of a time-related contribution (the standby time-related failure rate) and a cyclic, demand-related contribution (the demand stress failure probability). The latter is the probability contribution associated with failures that are caused by demanding, starting, or cycling the component, which include (but are not necessarily limited to) test-caused transients (discussed in Section A-2.3.6). Since the test-limited risk, RD, is associated with a failure occurring between tests, the failure rate that should be used in calculating the test-limited risk is the time-related standby stress failure rate. From the total number of failures on demand, the number of failures caused by standby stress and the number of failures from demand stresses can be partitioned by either an engineering analysis of failure causes or by a graphical method based on the relationship between the observed number of failures and the SFs from which the failures came. The test-caused contribution to risk is primarily composed of Rdown, the risk contribution caused by the unavailability of equipment from aligning the equipment away from its preferred position or state DG-1287, Appendix A, Page A-10 to conduct a test, when there is no automatic return to the preferred position. The additional data needed for estimating this parameter are the SF and the out-of-service time needed for each test. Dividing the failure probability into a time-related and cyclic demand-related contribution results in a lower test-limited risk because only part of the component’s failure rate is treated as time related. However, treating only part of the failure rate as time related when this is not the case underestimates the test-limited risk; therefore, such a breakdown of the failure rate should be justified through data or