Consideration for safety design of pipings in preparing the program of instrumentation

The safety design to be considered in the preparation of the program of instrumentation is described, taking a recent BWR nuclear power station as an example. The fundamental items of safety design include the assumption of single failure, the adoption of redundancy, the securing of physical independency and separation, the adoption of fail-safe and fool-proof design, the employment of highly reliable logic circuits, the automation of operation, and the provision of possibility of in-service testing. In system constitution, the securing of redundancy and the automation of operation must be considered against the design basis accident, while in steady operation, the indirect cooling system for station auxiliaries and the independence must be considered. In the system design specifically of emergency core cooling system, the possibility of in-service testing, the duplication of water source, the installation of minimum flow line and the prevention of reverse flow of reactor water should be considered. The provision of isolation valves, the restriction of outflow of primary fluid, the prevention of leakage between systems, the duplication of valves and the parallel arrangement of equipments should be considered around the reactor and auxiliary systems. Design conditions must be determined by assuming maximum operating conditions. The physical separation of equipments is the indispensable measure in system installation to avoid the common mode failure due to missile phenomenon, flooding or the like. This applies also to electrical systems and instrumentation and control systems. (Wakatsuki, Y.)