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:
undant components in a system (i.e., whether staggered or sequential testing is performed). The standard PRA quantification assumes that components follow no specific schedule and are randomly placed. Staggering the test times of components in different trains will reduce the test-limited risk contribution for the same SFs as compared to the PRA assumption. Conversely, if the tests are carried out sequentially, the test-limited risk will increase compared to the PRA assumptions. A-2.3.4 Parameters for Component Unavailability The component unavailabilities used in a PRA contain several relevant parameters for evaluating TS changes. These parameters should be delineated, as modeled, to facilitate evaluations to be conducted and their review by the regulatory authority. The following desirable parameters contribute to the estimated component unavailability: a. component failure rate, b. component test interval, c. maintenance/repair downtime contribution (i.e., maintenance frequency, downtime for scheduled and unscheduled maintenance), d. test downtime, if applicable, e. human errors following test or maintenance, if modeled, and f. separation of cyclic-demand versus standby time contribution, if modeled. A-2.3.5 Separating Demand and Standby Time Contributions to Unavailability Since the test-limited risk (typically defined as RD) is associated with a failure occurring between tests, the standby time-related failure rate should be used in calculating the test-limited risk. The standby time-related failure rate is associated with what could occur while the component is in standby between tests. Test-limited risk contributes to increases in risk associated with longer test intervals caused by the longer time to detect standby-stress failures. The time-related failure rate 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