Risk-Informed Regulation of Digitial Instrumentation and Controls
As discussed in the NRC's policy statement on probabilistic risk assessment (PRA), the agency supports the use of PRA in regulatory matters to the extent supported by state-of-the-art PRA methods and data. Since digital systems will play an increasingly important role in nuclear power plant control and safety systems, the need for risk assessment methods for digital systems is evident.
Accident Sequence Precursor Events
An accident sequence precursor (ASP) events database study has demonstrated the prevalence of embedded (digital) I&C components and their impact on plant safety. This study identified several ASP events that involved failure of digital controls that were embedded in larger plant systems (e.g., circuit breakers, transformers, and diesel generators). Because of the prevalence of digital I&C systems and their potential impact on plant safety, future risk-informed regulatory decisions are likely to require risk assessment of both freestanding and embedded digital systems.
The NRC is actively working to develop tools and methods to perform quantitative risk assessments of nuclear power plant digital systems. This information will assist the staff in evaluating proposed digital system applications to ensure that they do not result in an unacceptable increase in the frequency of occurrence of an accident or in the likelihood of occurrence of a malfunction of a structure, system, or component important to safety.
Objectives of Risk Assessment
The objectives of risk assessment are to
- identify failures that can occur,
- determine the impact of those failures, and
- quantify their frequency.
The NRC is investigating the use of methods, tools, and criteria to meet these three digital risk assessment objectives. This entails assessing the types and causes of failures that can occur in digital systems, characterizing the risk-importance of I&C systems (impact of digital failures on safety), developing digital reliability assessment methods (frequency of failures), and collecting and analyzing the data needed to support this work. The staff recognizes the potential difficulty in integrating digital systems into PRAs and in the practicality of using a PRA to assess digital systems.