Fault tree modeling of human error dependency in PSA

초록

In probabilistic safety assessment(PSA) of a nuclear power plant, a human failure event is one of the events that have high risk contribution. In addition, dependency between the events may increase the risk contribution. This is because the success or failure of a previous action can affect the subsequent action. In the case of the scenario in which more than two human failure events occur, human error dependency should be analyzed and reflected in PSA. For this reason, the probability of a human failure event may be different depending on the accident scenario or other human failure events. When the residual heat removal is unavailable after failure of makeup the coolant in an overdrain accident at low power and shutdown, feed and bleed operation may be needed to prevent core damage. There are two cases when feed and bleed is required after the failure of coolant makeup due to machinal failures and due to a human failure event. The human error probability of feed and bleed after a human failure event of the makeup may be higher because of the dependency. [1,2] Human error dependency has been reflected through the post-processing. There are several limitations in the existing method because the dependency is reflected after minimal cutsets are derived. Several minimal cutsets may be improperly truncated by the cut-off value. Propagation on a fault tree is also difficult. This study proposes a method of modeling the human error dependency on the fault tree. We also provide the applicable cases to model the human error dependency into the fault tree. This method can be practically applicable to a PSA model while supplementing the limitations of the existing method. © ESREL 2021. Published by Research Publishing, Singapore.

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