Intersystem Software Common Cause Failure Modeling of Digital Instrumentation and Control Systems

Abstract

The Instrumentation and Control (I&C) systems of nuclear power plants play a crucial role in monitoring, controlling, preventing, and mitigating accidents in the plant. Traditional analog I&C systems face various challenges such as aging, reduced accuracy, and noise distortion due to prolonged use. In contrast, digital I&C systems offer enhanced accuracy, maintainability, and superior computational capabilities, making them easier to operate and maintain with greater flexibility. Some nuclear power plants are upgrading analog systems to digital systems to safety and reliability, leveraging the advantages provided by digital systems. New reactor design improve also adopts digital I&C systems to enhance operational efficiency and safety. However, as the complexity of digital systems and interactions among subsystems increase, there is a potential for faults in the system. Issues like Common Cause Failures (CCF), especially in software, can pose a significant threat to safety. The U.S. Department of Energy's RISA pathway has performed a project for effective, licensable, and secure digital I&C technologies. This project proposed a risk assessment framework for digital I&C systems and presented a method for comprehensively evaluating CCF of digital I&C systems using the modified Beta Factor Model (BFM). This study utilizes the proposed method to analyze various coupling factors and conducts a case study on intersystem software CCF within digital systems.