Document: NUREG-0800
Document ID: 8dd882f4-a34f-4415-acd1-ebb441c72786
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1213/ML12132A481.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
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CFR Title: 

Content:
al emphasis should be placed on PRA modeling of novel and passive features in the design, as well as addressing issues related to those features, such as digital instrumentation and control, explosive (squib) valves, and the issue of T-H uncertainties.5 5 The issue of T-H uncertainties arises from the “passive” nature of safety-related systems used for accident mitigation. Passive safety systems rely on natural forces, such as gravity, to perform their functions. Such driving forces are small compared to those of pumped systems, and the uncertainty in their values, as 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 which are not predicted to lead to core damage by a best-estimate T-H analysis, may actually lead to core damage when T-H uncertainties are considered in the PRA models. 19.0-28 Draft Revision 3 – September 2012 3. The following guidelines for reviewing digital instrumentation and control (DI&C) system risk assessments are based on the lessons learned from previously accepted new reactor DI&C system PRA reviews. The review should consider the following steps, as applicable, to ensure that the risk contributions from DI&C, including software, are reflected adequately in the overall plant risk results: A. The level of review of the DI&C portion of the PRA may be limited due to limitations such as the lack of design details, lack of applicable data, and the lack of consensus in the technical community regarding acceptable modeling techniques for determining the risk significance of the DI&C system. The level of review should be proportional to the use of results and insights from the applicant’s DI&C risk assessment. B. The modeling of DI&C systems should include the identification of how DI&C systems can fail and what these failures can affect. The failure modes of DI&C systems are often