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Risk Assessment of Digital Systems

The current NRC digital I&C system licensing process is deterministic. In the 1997 National Research Council report on the digital I&C system in nuclear power plants, it was recommended, “The U.S. NRC should strive to develop methods for estimating failure probabilities of digital systems, including COTS [commercial off-the-shelf] software and hardware for use in probabilistic risk assessment.” The report also indicated, “These methods should include acceptance criteria, guidelines, and limitations for use and any needed rational and justification.” Additionally, the NRC PRA Policy Statement encourages the staff to risk-inform all regulatory reviews to the extent supported by the state of the art.

The NRC and the industry are interested in risk-informing digital safety system licensing reviews. The staff has been working over the past 2 years to develop risk and reliability methods needed to risk-inform digital system reviews. The Electric Power Research Institute (EPRI) has also proposed an approach to risk-inform the diversity and defense-in-depth analysis. More broadly than diversity and defense in depth, the NRC research program is designed to develop methods for modeling reliability and the risk of digital I&C.

One of the major challenges to risk-informing digital system reviews is developing a common method for modeling digital system reliability. The staff examined a number of reliability and risk methods that have been developed in other industries—such as aerospace, defense, and telecommunications—and has determined that some of the digital system risk modeling methods used in these industries can be adapted for use in the nuclear industry. The results of the first phase of the digital system risk research project were published as NUREG/CR-6901, “Current State of Reliability Modeling Methodologies for Digital Systems and Their Acceptance Criteria for Nuclear Power Plant Assessments." This report reviewed a number of reliability methods that have been used by other industries to model digital systems as well as the research that has been done to support their implementation in practical engineering analysis.

Based on the staff’s review of these techniques, together with available failure data, the NRC is evaluating several digital system modeling methods with the intent of establishing best practices for modeling digital systems in nuclear power plants. These methods include Markov modeling, dynamic flow graph methods, and traditional event tree/fault tree methods.

The staff plans to use the digital system reliability and risk analysis methods that are being developed and benchmarked as part of this research as one acceptable method that can be applied to current NRC guidance for using PRA in risk-informing licensing decisions (Regulatory Guide 1.174, “An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis”). The staff envisions developing regulatory guidance that will provide specific guidance for risk-informing digital system licensing decisions in a way similar to that of Regulatory Guide 1.177, “An Approach for Plant-Specific, Risk-Informed Decision making: Technical Specifications,” which provides specific guidance for risk-informing technical specification changes.

Draft regulatory guidance is scheduled for completion in late 2007, and final regulatory guidance should be completed in mid-2008.