Development of guidance for the review of new reactor digital insrumentation and control probabilistic risk assessmets

DI&C systems are complex combinations of hardware components and software (i.e., computer programs). This combination of complex hardware and software can result in the presence of faults and failure modes unique to DI&C systems. For DI&C systems, failures arise from the combination of a fault in the system in conjunction with a set of circumstances (e.g., a plant transient or accident) that satisfies the conditions required for the fault to be exercised. The nuclear industry has purposed to design and implement DI&C systems in new reactors that have a low probability of containing significant faults. In particular, the designers have attempted to reduce the likelihood of DI&C common cause failure. New reactors licensed under 10 CFR 52 are required to have a PRA (a design-specific PRA at the DC stage as well as site-specific PRA at the COL stage) and are reviewed to both the instrumentation and control and the probabilistic risk assessment (PRA) sections of the "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," guidance. However, due to data limitations and the lack of consensus modeling tools, the assessment of DI&C system risk for new plants has been limited to examining assumptions, performing sensitivity studies, and evaluating importance measure values. The NRC staff has developed interim guidance that provides acceptable methods for evaluating digital instrumentation and control system risk assessments. The primary purpose of this guidance is to provide information on how NRC reviewers should evaluate digital instrumentation and control (DI&C) system PRAs, including addressing inclusion of common cause failures in PRAs and uncertainty analysis associated with new reactor digital systems.