Today, probabilistic risk assessments (PRAs) at multi-unit nuclear power plants consider risk from each unit separately and do not formally consider combination events between the units. To gain an accurate view of a multi-unit site’s risk profile, the core damage frequency for the site, rather than the unit, should be considered. There are many types of events that could create a dependency between multiple units from a risk perspective. In order to effectively account for these risks in a multi-unit PRA, six main dependence classifications have been created: initiating events, shared connections, identical components, proximity dependencies, human dependencies, and organizational dependencies. This paper discusses these six classifications that could create dependence between multiple units. As a validation of the classification, this paper will also discuss multi-unit events that have occurred.

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