The fault tree as a tool in safety analysis in nuclear power plants

Modern safety analysis must be able to identify realistic failure modes based on realistic operation and system malfunction, demonstrate rigorously that adequate independence exists between a malfunctioning system and those other systems required to mitigate the effects of the malfunction, design adequate reliability into systems important to plant safety and to demonstrate rigorously that the design reliability is met in operation, and identify the realistic actions expected of the operator. Fault trees, which have proved to be a powerful tool to achieve these objectives, are inevitably large and must be computerized. However, the computerized system must be simple, must allow merging of branches developed independently, must provide for easy modification and the processing must be economical and easily accessible. A new system for displaying, plotting and analysing fault trees has been developed and implemented on a small computer at AECB to demonstrate the viability of the approach to designers, and to provide a tool to assess licensee's submissions on failure modes of support systems such as electrical, service water and air, and to assess reliability predictions for special safety systems. (author)