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:
. When the risk impact of a CT change is evaluated, the yearly risk impact that is calculated takes into account the outage frequency. A CT extension may imply that the maintenance of the component is improved, which may reduce the component’s failure rate and, consequently, reduce the frequency of outages needed for correcting degradations or failure. There are no experience data for the extended CT; therefore, the assumption should be made that both the DG-1287, Page 16 frequency of outage for corrective maintenance and the component’s failure rate remain the same. Here, the beneficial aspect of maintenance is not quantified, and this may give a slightly higher estimate of the yearly CT risk measure for the proposed CT. d. Often, CT extensions are requested to facilitate online (or at-power) preventive maintenance of safety-system components. The frequency and duration of the extension may be estimated and the risk impact from the resulting unavailability of such equipment can be calculated. e. When CTs of multiple safety system trains are extended, the likelihood of simultaneous outages of multiple components increases (resulting from combinations of failures, testing, and maintenances) because the increased duration increases the probability of the individual events that constitute the simultaneous multiple outages; hence, overlapping of routinely scheduled activities and random failures becomes more likely. The impact of such occurrences on the average plant risk (e.g., CDF) is small, but the conditional risk can be large. This issue is addressed as part of the implementation considerations (see Regulatory Position C.2.3.7). SF evaluations should consider the following assumptions: a. Surveillance tests usually are assumed to detect failures that have 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