**Related events:**

• **Category 1:** Consequential (or subsequent) events:

The events are causally related. An initial event, for example, an external hazard, results in another consequential event, for example, an internal hazard. Typical examples are seismic and consequential internal explosion and/or fire, internal fire and consequential internal flooding, external flooding and consequential high energy arcing fault (HEAF) of a component and subsequent fire.

• **Category 2:** Correlated events:

Two or more events, at least one of them representing a hazard, do occur as a result from a common cause. The common cause can be any anticipated event including external hazards. The two or more events correlated by this common cause could even occur simultaneously<sup>1</sup> .

<sup>1 &</sup>quot;Simultaneous" here does not mean that the events occur exactly at the same time but that the second event occurs before the previous one has been completely mitigated.

Typical examples are electromagnetic interference (EMI) as common cause for a station blackout (SBO) and an internal fire as two correlated events or Tsunami as common cause for external flooding, internal flooding, and internal fire as three correlated events.

• **Category 3:** Unrelated events:

An initial event, for example, a (external or internal) hazard occurs independently from but simultaneously<sup>1</sup> to a hazard without any common cause. Typical examples are external flooding and independent internal fire or explosion, seismic event, and independent internal fire.

For each category of event combinations with hazards involved those combinations, which can occur site-specifically and according to the design and protection features of the facility or plant to be analyzed, have to be identified and undergo a systematic screening. In the frame of the assessment of the contribution of natural hazards to risk, some hazard classes cannot be combined with natural hazards (hazard classes A to H) depending on the category of combinations. This limits the amount of principally possible combinations significantly.
