Dependency analysis method for human failure events in multi-unit probabilistic safety assessments

Dependency between human failure events (HFEs) is often analyzed as a part of the conventional human reliability analysis (HRA) process for nuclear power plants (NPPs). None of the existing methods have considered an application to multi-unit (MU) scenarios. In this study, we introduce a novel approach to evaluate the dependencies between HFEs in cases of MU event scenarios. We focus on developing a set of MU HFE dependency evaluation elements and their evaluation criteria based on the framework of the single-unit evaluation elements that have been utilized in the HRA practice for NPPs. The unique MU HFE dependency evaluation elements are defined, and we present a new dependency evaluation tree for qualitative analysis. In addition, important cases are provided to demonstrate the application of the proposed method, including four MU initiating events: MU loss of offsite power, MU loss of condenser vacuum, MU general transient, and MU loss of ultimate heat sink. Quantitative analysis with several illustrative HFEs in MU probabilistic safety assessment (PSA) cutsets are also presented. This method fills the gap of a HFE dependency analysis method in the case of MU PSA.