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:
. Such evaluations can be considered like those evaluations made directly using PRA models, but they should satisfy the following conditions: DG-1287, Page 14 (1) Failures within the system should not affect any other system or component failure. (2) The effect of system failure should not influence any initiating event frequency (or it should have a minimal or negligible effect). (3) The system should not share components with another system. c. When bounding evaluations are performed assuming any failure in the system as a system failure, the calculated risk impacts for TS changes are expected to be overestimated. The corresponding changes that may be acceptable will also be fewer than those that could have been justified using a detailed model. When considering the incorporation of non-PRA factors, this perspective should be kept, while at the same time considering the lack of a detailed model. Here also, the above three conditions discussed for the previous case apply. In some cases, since the risk-informed evaluation will be limited and some misestimation of the risk may have been incorporated, nonrisk-related engineering considerations gain importance in the overall decision. In such cases, arguments for the change also should be for small increments from current requirements. 2.3.3.2 Modeling of Initiating Events The PRA explicitly models (i.e., use of detailed fault tree models in the PRA) some initiating events resulting from support system failure (e.g., service water, component cooling water, instrument air). Any TS change for these systems will affect the corresponding initiating event frequency as well as the system unavailability and availability of other supported systems. The effect of TS changes on these initiating event frequencies should be considered. Some test and maintenance activities can contribute to some transients. Initiating event frequencies used in the PRA do not typically separate out this contribution, but such a separation may be