In order to enhance the safety of new advanced reactors, optimization of the design of the implemented passive systems is required. The development of a model, aiming at assessing the probability of the flow being less than required for natural circulation, is needed, for the reliability-based design approach of a thermal-hydraulic (T-H) passive system.The reliability mode l is implemented in the form of the functional failure concept, i.e., the possibility that the load will exceed the capacity in a reliability physics framework. Thus, in order to evaluate the system probability to accomplish the requested mission to achieve a generic safety function (e.g. decay heat removal), the well known stress-strength interference model drawn from mechanical reliability is introduced, in terms of system relevant performance parameter.The strength is represented by the functional requirement and is probabilistically compared to the system state, i.e., the stress. The reliability of the system is evaluated in terms of the safety margin corresponding to the limit state function or performance function according to a defined safety criterion, for the determination of the allowable window far the system performance parameter.
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