Software safety assessment and the Sizewell B applications

The Sizewell B PWR has two diverse protection systems which initiate automatic reactor trip and engineered safety features (ESF) needed for any fault, by detecting if measured plant signals are outside defined limits. Both systems have four redundant channels with two-out-of-four voting for each action. The primary protection system (PPS) is a computer based system and protects the reactor for all design basis faults. The station also has a computer based integrated system for centralised operations (ISCO), which includes self-contained high integrity computer system (HICS) sections. The authors describe the processes of assessment carried out for Nuclear Electric (NE), of the software provided by Westinghouse, as suppliers of the PPS and HICS. NE conducted extensive reviews and analysis of the software, in the course of establishing the safety case