Failure modes taxonomy for digital I&C systems: Common framework for PSA and I&C experts

Failure modes taxonomy is a framework of describing, classifying and naming failure modes associated with a system. Main uses of failure modes taxonomies are in the performance of reliability analyses and in the collection of operating experience (failure data) of technological systems. The Nordic NKS/NPSAG/SAFIR financed project DIGREL and the parallel OECD/NEA Working Group RISK activity DIGREL aim at developing technically sound and feasible failure modes taxonomy for reliability assessment of digital I&C systems for probabilistic safety assessment (PSA), and to provide guidelines on the use of taxonomy in modelling, data collection and quantification of digital I&C reliability. The scope of the taxonomy will include both protection and control systems of a nuclear power plant (with primary focus on protection systems) and both hardware and software related failure modes. The taxonomy will be developed jointly by PSA and I&C experts which have slightly different views and needs on defining the failure modes. The PSA experts' perspective follows the needs of PSA modelling in order to capture relevant dependencies and to find justifiable reliability parameters. I&C experts are focused on failure mechanisms and their recovery means, e.g. V&V measures. An important aspect in the development of the taxonomy is for PSA and I&C experts to define the "meeting point" for the two perspectives. The paper will describe results of the DIGREL activity so far and discuss the commonalities and differences in PSA and I&C experts' perspectives on failure modes.