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:
Minimal Cutsets As indicated, a PRA computes the yearly CT risk contribution to the yearly CDF. Basically, the yearly CT risk contribution is the sum of the minimal cutset contributions containing the component-down unavailability (typically for maintenance) qm, d f qm ⋅ = where f is the downtime frequency and d is the downtime associated with the CT. The downtime d usually is estimated as an average downtime associated with the CT. If the minimal cutsets sufficiently cover the downed unavailability, those that contain the downed unavailability, qm, can be summed to give the yearly CT risk contribution, Ry. A-1.3.1.3 Using the Probabilistic Risk Assessment to Determine the Test-Limited Risk Contribution The PRA can be used to calculate the increase in the risk-level ΔR and to obtain the component unavailability, q, which are the contributing factors in calculating the test-limited risk contribution. The preceding and following sections discuss considerations involved in calculating R1 and R0 to obtain ΔR. When the effect of change in SF for one or more components is being evaluated, the PRA can directly calculate the change in the risk measure (e.g., in the CDF). The calculation of PRA results, when changed SFs are included, incorporates interactions among the SFs. The differences between the results (i.e., CDF when the SFs are changed from the baseline CDF) provide the test-limited risk contribution for changing the SFs. Such a calculation should include appropriately modified contributions of CCFs. The common-failure terms modeled as a function of the SF should be modified to reflect the new SF. Typically, CCFs are modeled using a β-factor or multiple Greek letter model when the CCF of multiple components is a function of the SF. When changing SFs, care should be taken to change this term within the common-cause contribution. The common cause of failing multiple components resulting from human error following a test is not a function of the SF, but it may be