Document: NUREG-0800
Document ID: dd4f84a9-8976-44c4-b631-d7c0254f4efc
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1416/ML14161A594.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
echnical acceptability. If the applicant’s justification fails to provide the staff with an appropriate level of confidence in the models, results, and insights, the staff should conduct an audit of the applicant’s PRA against the technical elements described in RG 1.200 to determine the PRA technical adequacy. Design-Specific PRA (Procedures Specific to Passive Designs) 1. The issue of T-H uncertainties in passive plant designs arises from the passive nature of the safety-related systems used for accident mitigation. Passive safety systems rely on natural forces, such as gravity, to perform their safety functions. Such driving forces are small compared to those of pumped systems, and the uncertainty in their values, as 19.0-24 Draft Revision 3 –November 2014 predicted by a best-estimate T-H analysis, can be of comparable magnitude to the predicted values themselves. Therefore, some accident sequences with a frequency high enough to impact results, but not predicted to lead to core damage by a best- estimate T-H analysis, may actually lead to core damage when PRA models consider T-H uncertainties. One approach to addressing this issue is to perform sensitivity studies to see the affect of assuming bounding values for T-H parameters on success criteria and performing studies of the sensitivity of changes in success criteria on CDF. A. The reviewer assures that the applicant has (1) identified all key T-H parameters that could affect the reliability of a passive system and introduce uncertainty into the determination of success criteria, and (2) accounted for the uncertainty in the analyses that establish the success criteria. B. The reviewer examines the results of any sensitivity studies performed by the applicant and the choice of T-H accident analysis codes used to perform such studies. Applicants frequently use the Modular Accident Analysis Program (MAAP) code for such studies. The staff is aware of T-H modeling issues with the code that could compromise