*3.2.2. Quantitative screening of hazard combinations*

In the example of a German nuclear power plant site, given cutoff values from the German regulation [4] have been applied to the occurrence frequency and to the damage frequency.

The qualitative screening for category 1 combinations provides the following result for causally related combinations at the reference site with the hydrological hazards B2, B3, B4, B6a, B8, and B9a: Combinations of these hazards with E6 (biological flotsam) and F1 (subaerial slope instability) have been screened out quantitatively. Therefore, no category 1 combinations remain after the quantitative screening; higher order combinations are also not to be assumed.

The following category 2 combinations that remain after qualitative screening for the reference plant site have been analyzed: A meteorite fall (D2) can cause correlations of I2 (internal flooding) with B6a, B8, or B9a. I2 can also occur together with B6a, B8, or B9a as consequence of man-made explosions (Z1, Z4, Z6, Z11, or Z13). Resulting from a common cause such as a thunderstorm precipitation (C1) or F1 (subaerial slope instability) can be observed correlated with B2 or B4. All these correlations have been excluded quantitatively for the reference facility.

For category 3 combinations B2, B3, B4, B6a, B8, or B9a have to be assumed to occur independent from other hazards. Such combinations have only to be analyzed, if their occurrence frequencies exceed a given cut-off value under consideration of the durations of the individual hazards. According to this argumentation, in the example of screening for hydrological hazards for a given German site, only combinations of B2 with B3 or B4 finally remain for further detailed risk analysis.

It could be demonstrated that the remaining number of hazard combinations is significantly lower after qualitative and quantitative screening of hazard combinations.
