A quantitative evaluation of reliability of passive systems within probabilistic safety assessment framework for VHTR

This article presents a quantitative evaluation of the reliability of passive systems (RoPS) within the probabilistic safety assessment (PSA) framework for very high temperature reactors (VHTR). VHTRs have unfavorable features in regard to defining a robust failure state. From the viewpoint of PSA, the evaluation of the RoPS as a part of VHTR's PSA should carefully consider the correct status of a passive system in order to resolve these unfavorable features. This article focuses on the application of multiple states criteria to determine the status of a passive system. Two approaches, i.e., the exceedance probability (EP) model and the stress-strength interference (SSI) model were proposed for the multiple states of the system. A feasibility study has examined the basic features of the proposed approaches by using the reactor cavity cooling system (RCCS) for Korean VHTR. The primary condition for the usefulness of the proposed approaches is that sufficient information should be provided in order to determine the system strengths for the multiple states. With regard to the engineering practice, the EP approach for the multiple states can provide a practical solution concerning the evaluation of the RoPS for VHTR's PSA.