Safety case of digital protection systems focusing on safety-critical software failure in NPPs

The safety systems of nuclear power plants are manipulated by the digital instrumentation and control systems, which provide the control and monitoring functions of the various and diverse components and equipment that are essential to maintaining safe operation. Software failure can be defined as not successfully performing a specified/intended function or performing unintended actions. It is known that software failure is one of the most important features of digital systems. A study also identified that the estimation of software failure probability is an important factor. The sensitivity study in the study on the digital reactor protection system (RPS) showed the relationship of system unavailability and software failure probability. RPS software safety case can be defined based on each type of software failure causes in consideration of failure characteristics. If suggested evidence of each claim is provided, it can be said that the risk from RPS software is sufficiently low.