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There is so far no consensus on how to develop a reliability model of safety-related digitalinstrumentation and control (I&C) in a probabilistic safety assessment (PSA) of a nuclear powerplant. The objective of this thesis is to evaluate different approaches to model digital protectionsystems in a PSA for a nuclear power plant. This is accomplished by the development of a fault treemodel of the digital protection system for a fictive and simplified nuclear power plant, that act as areference model to be used for evaluation of different design alternatives and modelling principles.Common cause failures and spurious actuation signals are the major contributors to scenariosresulting in a core damage. A PSA model has to be sufficiently detailed in order for this to berepresented in the results. The impact on results such as core damage frequency and importance ofminimal cut sets from different fail-safe, voting logic and signal validation designs are significant,too. To further examine the differences between I&C designs and significance of different PSAmodelling solutions, the degree of realism of the example should be increased. This rapidlyintroduces complexity to the models resulting in a model that is more difficult to review and resultsthat is more difficult to interpret and even much simplified models tend to get rather complex.

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