The main thrust of the Probabilistic Riak Assessment (PRA) model is to assess the spectrum possible scenarios (sequences of events) that can lead underirable consequences. However, there is no guarantee that all possible scenarios have been iden5tified and propoerly assessed. This loack of completeness introduces an uncertainty in the results an conclusions of the analysis. Even for scenarios, which have been identified, the event sequence a nd system logic models do not precisely represent reality. There are uncertainties introduced by the relative inadquacy of the conceptual models, the mathematical models, the numerical approximations, the codings errors and the computational limits.
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