Guidelines for the Use of Function Block Diagram in Reactor Protection Systems

Making software dependable is one of most important aspects in safety-critical system such as a nuclear power plant. Dependable programming techniques to get rid of undependable properties, such as ambiguity, wrong uses of elements, discordance, etc., help engineers reduce the faults in programs. This paper proposes the practice guidelines for function block diagram (FBD) which is a programming language for programmable logic controllers (PLC) widely used in industry. The guidelines show that what cases cause undependable properties and how the properties should be eliminated to be dependable ones in FBD programs. The paper introduces the application of guidelines to the trip (shutdown) logic of bistable processor (BP) in reactor protection system (RPS) developed in the Korea Nuclear Instrumentation and Control System R&D Center (KNICS). The application describes that the guidelines eliminates undependable properties in the trip logic.