Document: NUREG-0800
Document ID: b8d2b8f5-432a-4c0a-81ab-20231f7d0f28
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1508/ML15089A068.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
s related to those features, such as DI&C, explosive (squib) valves, and the issue of T-H uncertainties. 3. The following guidelines for reviewing 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 identified by the performance of failure modes and effects analyses (FMEA). It is difficult to define DI&C system failure modes especially for software because they occur in various ways depending on specific applications. Also, failure modes, causes, or effects often are intertwined or defined ambiguously, and sometimes overlap or are contradictory. The reviewer should review the depth of the FMEA or other hazard analysis techniques employed by the applicant to ensure the process employed is systematic and comprehensive in its identification of failure modes. The PRA reviewer should work with the I&C reviewer to evaluate the methodology and results provided by the applicant. Examine applicant documentation to ensure that the most significant failure modes of the DI&C are documented with a description of the sequence of events that need to take place to fail the system. The sequence of events should realistically represent