The complexity of computer programs controlling critical systems in nuclear plants makes it very hard to ensure safety, an authoritative Government committee warned this week.
The Nuclear Safety Advisory Committee (Nusac) said that while the use of computers in safety-critical applications had been successful so far, this was ‘far from sufficient’ to conclude that any systems were free from design faults ‘that could cause them to fail, perhaps catastrophically’.
The long-awaited report from Nusac initiated in 1995 following problems in developing a computerised primary protection system (PPS) for the Sizewell B reactor said problems arose from inherent ‘bugs’ in programs of any size, their discrete rather than continuous behaviour and, above all, their complexity.
The PPS at Sizewell B, for instance, has 100,000 lines of code a figure the report contrasts with the 10,000 lines of (usually) easily understandable language found in the average modern novel.
The report says this results in great difficulties of validation and understanding: ‘It is often the case that no single person can claim to understand a program completely, and this brings with it uncertainty about the properties of the program particularly its reliability and safety.’
The report makes 19 recommendations, covering regulatory practice, safety cases, system design and software engineering, standards and research.