Dynamical reliability of the passive system in the very high temperature gas cooled reactor

A dynamic safety assessment has been developed for the passive system in the very high temperature gas cooled reactor (VHTR), when the operational data are insufficient. It was required to make use of the characteristics of the reactor in order to compensate for the data shortage and to treat the propagation of incidents. Therefore, this paper focuses on the failure frequency construction of the basic events and the advanced method of treating the propagation. The mass flow rate caused by the natural circulation in the passive system is related to the fuel temperature which affects the failure fraction of the fuel. These features are utilized for finding the failure frequency of the basic event. The non-linear string logic is used due to the simple and tractable algorithm of the passive system instead of the tree concept which is used in the event-fault tree based decision making. The time feedback is applied to the string concept, where the time weighting is adjusted by the operator's judgment. Results are obtained for four cases. Among them, two cases are non-linear transition features of the events using feedback. The other two cases are based on linear propagations, which construct the characteristics of the dynamic resistance-stress method (DRSM). Using the string algorithm, one can successfully perform safety assessment for any other advanced reactor such as the VHTR.