**3. Hazards screening**

For a systematic assessment of the contribution of natural hazards to risk, only the hazard

When combining hazards with other anticipated events, three different categories of combi-

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 compo-

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>

1 "Simultaneous" here does not mean that the events occur exactly at the same time but that the second event occurs

.

classes A to H have to be considered as initially occurring individual hazards.

**2.2. Categories of hazard combinations**

**Hazard Type of individual internal hazard**

132 Probabilistic Modeling in System Engineering

I4 Pipe breaks (whip/jet effects, flooding) I5a Heavy load drop/falling objects

I5b Collapse of structural building elements

I9 Electromagnetic interference (EMI)

I11 Release of hazardous substances I12 On-site excavation and construction

**Table 10.** Class I hazards according to [2].

I6 On-site collision of vehicles

I7 Internal explosion I8 Multi-unit impact

I10 Missiles

I3 Component failure (including high energy faults)

I13 Underground high-voltage Eddy currents (on-site)

I1 Internal fire I2 Internal flooding

**Related events:**

nent and subsequent fire.

• **Category 2:** Correlated events:

before the previous one has been completely mitigated.

nations need to be distinguished (see also [1]):

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

For limiting further detailed analyses only to those hazards and hazard combinations, which can occur at the site and in the facility under investigation, a systematic screening is needed. In Germany, a clearly structured, systematic approach for hazards identification and screening has been developed in the recent past by GRS for probabilistic risk assessment of nuclear power plant sites with respect to hazards [2]. This approach uses for the collection and processing of generic as well as site- and plant-specific information needed for screening and detailed analysis an analytical tool called *Hazards Library*. Based on the information and data available in general and the plant under investigation, a step-wise screening with a qualitative and a quantitative screening step, first for individual hazards and, based on the results of the qualitative screening, afterwards also for hazard combinations, is done. The screening can be performed semi-automatically based on questions to be answered for the qualitative screening and applying preselected quantitative criteria for the quantitative screening. A schematic overview of the screening approach is given in **Figure 1**.
