Reliability assessment method for NPP digital I&C systems considering the effect of automatic periodic tests

Since digital technologies have been improved, the analog systems in nuclear power plants (NPPs) have been replaced with digital systems. Recently, new NPPs have adapted various kinds of digital instrumentation and control (I&C) systems. Even though digital I&C systems have various fault-tolerant techniques for enhancing the system availability and safety compared to conventional analog I&C systems, the effects of these fault-tolerant techniques on system safety have not been properly considered yet in most probabilistic safety assessment models. Therefore, it is necessary to develop the safety evaluation method for digital I&C systems with consideration of fault-tolerant techniques. Among the various issues in the safety model for digital I&C systems, one of the important issues is how to exclude the duplicated effect of fault-tolerant techniques implemented at each hierarchy level of the system. The exact relation between faults and fault-tolerant techniques should be identified in order to exclude this duplicated effect. In this work, the relation between faults and fault-tolerant techniques are identified using fault injection experiments. As an application, the proposed method was applied to a module of a digital reactor protection system.