Compliance with Standards or Claim-based Justification? The Interplay and Complementarity of the Approaches for Nuclear Software-based Systems

摘要

The control and protection of nuclear power plants has become increasingly dependent on the use of computers. The UK nuclear regulatory regime requires that a safety case be developed to justify and communicate their safety. There are several ways of constructing such a safety case. In the past, safety justifications tended to be standards-based - compliance to accepted practice was deemed to imply adequate safety. Over the last 20 years, there has been a trend towards an explicit claim-based approach, where specific safety claims are supported by arguments and evidence at progressively more detailed levels. These approaches are not mutually exclusive, and a combination can be used to support a safety justification. In fact, for the most critical systems it can be argued that a safety case should consider both aspects. For less critical systems, one might believe that one approach would suffice. This paper discusses software-based systems with only a modest integrity requirement, and the interplay of the two approaches. It describes our experience with justifying such systems for the nuclear industry, and it claims that there are a number of benefits of taking both approaches together.