Nordic experience and experiments of modeling digital I&C systems in PSA

• The OECD/NEA CSNI Working Group on Risk Assessment (WGRisk) has set up a task group called DIGREL to develop a taxonomy of failure modes of digital components for the purposes of probabilistic safety assessment (PSA). A parallel Nordic activity carried out a prestudy where a comparison of Nordic experiences and a literature review were performed. The study showed a wide range of approaches and solutions to the challenges given by digital I&C, and also indicated that no state-of-the-art currently exists. 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. The example shows that even rather simple I&C design leads to rather complex model despite of the fact that many things have been simplified and only a few protection signals are considered. One lesson from the example is that the Alpha factor model should be used to model CCF: s instead of the Beta factor model. Two options were developed to the comparison of different fail-safe principles. The role of detectable and undetectable failure modes with respect to the failed versus spurious actuations can be clearly seen in the results, showing the importance to model these features in PSA.

Nyckelord

Digital I&C

Failure mode

Probabilistic safety assessment

Reliability analysis

Taxonomy

Publikations- och innehållstyp

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