Document: NRC Regulatory Guide
Document ID: 47b09be1-4bf8-45f9-a099-7fed871c09bd
Document Type: regulatory_guide
Title: Plant-Specific, Risk-Informed Decisionmaking: Inservice Testing (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2114/ML21140A055.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.175
CFR Part: 
CFR Title: 

Content:
nsideration. As discussed in Section 5 of NUREG/CR-6141, “Handbook of Methods for Risk-Based Analyses of Technical Specifications,” issued December 1994 (Ref. 20), the total failure probability of a component consists of a standby time-related contribution and a cyclic demand-related contribution. The cyclic demand-related contribution is associated with failures that are caused by stresses imposed on the component by demanding, starting, or cycling the component, such as electrical and mechanical stresses occurring when the component is demanded. The standby time-related contribution is associated with failures occurring while the component is in standby between tests related to deterioration and events that occur over time, such as from corrosion or erosion. In Section 5 of NUREG/CR-6141, the standby time- related contribution of the component failure probability is expressed in terms of a constant standby failure rate for transition to the failed state between tests, test interval length, and test strategy (e.g., independent testing, sequential testing, staggered testing). The total failure probability of the component is assumed to be the standby time-related contribution (i.e., no cyclic demand-related contribution); and therefore, the total failure probability scales linearly with test interval length (i.e., doubles when test interval doubles). This assumption is conservative in that it tends to overstate the effect of test interval extension leading to an overestimate of the associated change in risk. This assumption is therefore acceptable to the NRC staff. It should be noted that the total failure probability has a cyclic demand- related contribution, such that more realistic modeling in this area (i.e., dividing the total failure probability into a standby time-related and cyclic demand-related contribution) could be used to support development of the proposed RI-IST program. In this case, such a breakdown of the failure rate should be justified