*3.1.2. Quantitative screening of individual hazards*

The quantitative screening of individual natural hazards needs predefined quantitative criteria for screening out hazards by occurrence frequency or damage frequency. Such criteria are either available in the national or international regulation (e.g., for nuclear power plants, quantitative screening criteria by the regulatory bodies in charge of nuclear oversight are available), or conservative (pessimistic) cut-off criteria have to be defined for the facility to be investigated based on best practices.

For those hazards, for which the quantitative screening step needs to be carried out, the ranges of their occurrence frequencies have to be conservatively estimated. These are compared to cut-off frequency value corresponding to the screening criterion applied by the analyst.

Depending on the design of the facility with its protection measures and the corresponding safety margins for those hazards not screened out by frequency, a decision needs to be taken; one must decide for which hazards a rough risk estimate is sufficient and for which a detailed probabilistic analysis is needed.

For this purpose, the design requirements (national or international ones, for example, by the European Community) and their implementation at the site, for which the risk assessment shall be carried out, together with the site- and plant-specific boundary conditions and precautionary provisions against hazards impact need to be considered.

In case of the facility, for which the screening approach has been verified, the design against natural hazards such as external flooding covers events occurring once in 10,000 years corresponding to an occurrence frequency of 10−4 per year. Less frequent events as well as events with an occurrence frequency close to the design threshold but a damage probability of more than one order of magnitude lower can be screened out quantitatively.

The screening of hydrological (Class B) hazards for the reference site provides the result that B6, B8, and B9a can be screened out and only B2 "flash flood (torrent) by local extreme precipitation", B3 "flooding by melting snow" and B4 "flooding by extreme precipitation outside the plant boundary" remain for more detailed risk assessment. With respect to meteorological hazards (Class C), only C16 "high wind" remains at least for a rough analysis. Individual biological hazards are also screened out by frequency.

Those individual hazards screened out are stored in a list *L***0,individual**, those remaining after screening have to be considered for risk assessment and are stored, depending on their damage frequencies either in a list *L***rough,individual** for only rough risk estimates or in a list *L***detail,individual** for detailed analyses.
