Document: NRC Regulatory Guide
Document ID: 82659041-98b0-4721-b25d-c4fb2ea394d0
Document Type: regulatory_guide
Title: An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1635/ML16358A153.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.174
CFR Part: 
CFR Title: 

Content:
ndication of the changes that are generally acceptable. Furthermore, the approximate nature of PRA models as discussed below and the state-of-knowledge, or epistemic, uncertainties associated with PRA calculations preclude a definitive decision with respect to the region in which the application belongs based purely on the numerical results. However, licensees are not granted the same discretion when incorporating these guidelines by reference into other programs (e.g., Technical Specification Task Force (TSTF) Traveler TSTF-505, "Provide Risk-Informed Extended Completion Times - RITSTF Initiative 4b", 10 CFR 50.69). For example, a licensee may use its approved 10 CFR 50.69 program to re-categorize additional systems without prior NRC approval provided that the increase in risk meets the acceptance guidelines in this guide. In this context the licensee needs to treat the guidelines as hard criteria and is not allowed to consider the acceptance guidelines as met when the values are even slightly exceeded. The intent of comparing the PRA results with the acceptance guidelines is to demonstrate with reasonable assurance that Principle 4 (i.e., proposed increases in CDF or LERF are small and are consistent with the Commission’s Safety Goal Policy Statement) is being met. An important point to remember is that a PRA models the continuum of possible plant states in a discrete way, and are, by their very nature, an approximate model of the world. This results in some aspects of the “world” not being addressed except in a bounding way (e.g., different realizations of an accident sequence corresponding to different LOCA sizes, within a category, are treated by assuming a bounding LOCA), with the time of failure of an operating component assumed to occur at the moment of demand. These approximations introduce conservative or non-conservative biases into the results. The degree of conservatism or non- DG-1285, Page 31 conservatism could in principle be explored by