Guidelines for reliability analysis of digital systems in PSA context:Phase 3 Status Report

To assess the risk of nuclear power plant operation and to determine the risk impact of digital systems, there is a need to quantitatively assess the reliability of the digital systems in a justifiable manner. In 2007, the OECD/NEA CSNI directed the Working Group on Risk Assessment (WGRisk) to set up a task group to coordinate an activity in this field. One of the recommendations was to develop a taxonomy of failure modes of digital components for the purposes of probabilistic safety assessment (PSA), resulting in a follow-up task group called DIGREL. The taxonomy will be the basis of future modelling and quantification efforts. It will also help define a structure for data collection and to review PSA studies. This an interim report of the project. A draft guidelines document on the failure modes taxonomy has been developed. The taxonomy is rather complete covering all levels from the system level down to module and basic component level failure modes, including hardware and software aspects. In a parallel Nordic activity, an existing simplified PSA model has been complemented with fault tree models for a four-redundant distributed protection system in order to study and demonstrate the effect of design features and modelling approaches. The model has been used to test the effect of CCF modelling, fail-safe principle and voting logic. A comparison has been made between unit-level and module-level modelling