Finnish activities to develop safety assessment practices for programmable digital systems in nuclear power plants

Finnish utility companies Imatran Voima Oy (IVO) and Teollisuuden Voima Oy (TVO) and the licensing authority, the Finnish Centre for Radiation and Nuclear Safety (STUK), are preparing for a new nuclear power plant in Finland. Plant vendors are proposing programmable digital automation systems as well for safety related as for operational I and C systems in this new unit. Also in existing plant units the replacement of certain old analog systems with state-of-the-art digital ones will become necessary in the years to come. Licensing of programmable systems for safety critical applications require a new approach due to the special properties and failure modes of these systems. The major difficulties seem to be in the assessment and quantification of software reliability. The Technical Research Centre of Finland has in co-operation with the authority and the utilities conducted research to develop domestically applicable licensing requirements and practices. International standards, guidelines, licensing practices, software reliability assessment methods, quality metrics and quantitative methods have been analyzed in order to specify national licensing requirements. The paper will describe and discuss the problems of safety assessment of programmable systems and software of safety systems. Also factors influencing reliability of automation software as well as some of the available reliability assessment methods will be discussed. (author). 16 refs