Diversity requirements for safety critical software-based automation systems

System vendors nowadays propose software-based systems even for the most critical safety functions in nuclear power plants. Due to the nature and mechanisms of influence of software faults new methods are needed for the safety and reliability evaluation of these systems. In the research project "Programmable atuomation systems in nuclear power plants (OHA)", financed together by the Radiation and Nuclear Safety Authority (STUK), the Ministry of Trade and Industry and the Technical Research Centre of Finland (VTT), various safety assessment methods and tools for software based systems are developed and evaluated. A well known fact is that it in practice is almost impossible to produce totally fault-free software. Software faults are design faults introduced in the systems during the design and implementation process. Redundant channels containg identical software (with its faults) may therefore fail simultaneously i.e. the softwre faults can be a source of Common Cause Failures (CCF's). In safety critcal applications some kind of diversity is usually required to protect the systems safety functions against Common Cause protect the systems safety functions against Common Cause Failures. The diversity can be realized at harware, software of functional level or as a Common Cause Failures. The diversity can be realized at hardware, software or functional level or as a combination of these. For licensing purposes it is important to understand the influence of different approaches on the system reliability and safety and to adjust the diversity requirements to a proper level. This report firstly discusses the (common cause) failure mechanisms in software-based systems, then defines fault-tolerant system architectures to avoid common cause failures, then studies the various alternatives to apply diversity and their influence on system reliability. Finally, a method for the assessment of diversity is described. Other recently published reports in OHA-report series handle the statistical reliability assessment of software based (STUK-YTO-TR 119), usage models in reliability assessment of software-based systems (STUK-YTO-TR 128) and handling of programmable automation in plant PSA-studies (STUK-YTO TR 129).