Effect Analysis of Faults in Digital I&C Systems of Nuclear Power Plants

A reliability analysis of digital instrumentation and control (I&C) systems in nuclear power plants has been intoriduced as one of the important elements of a probabilistic safety assessment because of the unique characteristics of digital I&C systems. Digital I&C systems have various features distinguishable from those of analog I&C systems such as software and fault-tolerant techniques. In this work, the faults in a digital I&C system were analyzed and a model for representing the effect of the faults wa developed. First, the effects of the faults in a system were analyzed using fault injection experiments. A software-omplemented fault inject technique in which faults can be injected into the memory was used on the annumption that all faults in a system are reflected in the faults in the memory. In the experiment, the effect of a nfault on the system output was observed. In addition, the success or fault three model for representing that a fault is propagated to the system output was developled. With the model, it can be identicated how a fault is propagated to the output or why a fault is not detected by fault-tolerant techniques. Based on the analysis results of the proposed method, it is possible to not only evaluate the system reliability but also identify weak points of fault-tolerant techniques by indentifying undetected faults. The result can be reflect in the designs to improve the capability of fault-tolerant techniques