**3.1. Individual hazards screening**

The first step is the identification of those hazards, which cannot be directly excluded as practically impossible for the site being analyzed. For the assessment of natural hazards, in this first step, the hazard classes Z and I can be excluded as non-natural hazards.

For a given site, for example a riverine site in central Europe far from any coastal and/or tidal influence in an area with relatively high seismicity and no volcanic history, a variety of natural hazards can be excluded.

• Biological hazards: E6; • Geological hazards: F6.

quantitative screening step needs to be carried out.

*3.1.2. Quantitative screening of individual hazards*

investigated based on best practices.

probabilistic analysis is needed.

for detailed analyses.

For these, individual natural hazards remaining after the qualitative screening, the second,

Natural Hazards: Systematic Assessment of Their Contribution to Risk and Their Consequences

http://dx.doi.org/10.5772/intechopen.76503

135

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

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

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 pre-

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

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

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**

cautionary provisions against hazards impact need to be considered.

than one order of magnitude lower can be screened out quantitatively.

biological hazards are also screened out by frequency.

**Figure 1.** Overview of the stepwise approach for screening of hazards and hazard combinations, from [1].

#### *3.1.1. Qualitative screening of individual hazards*

*The qualitative screening* of individual hazards, which is mainly based on information available for the site being analyzed and from relevant operating experience, provides a list of sitespecific remaining individual hazards, which cannot be physically excluded. Some hazards can be easily screened out from further analysis because of the general conditions not being met at the site, such as hurricane or tropical cyclone, which do only occur in areas with tropic or sub-tropic climate, or sandstorms, which cannot be assumed based on results of detailed analyses (e.g., for siting and design of building structures) being available for the site ground. In case of a plant site on rock, several hazards such as sinkholes can be easily screened out.

As a result, the following individual hazards remain for the site being investigated after qualitative screening:

Seismotectonic hazards: A1, A3, A5;


*3.1.1. Qualitative screening of individual hazards*

134 Probabilistic Modeling in System Engineering

qualitative screening:

C24, C25, C27;

Seismotectonic hazards: A1, A3, A5;

• Hydrological hazards: B2, B3, B4, B6a, B8, B9a;

*The qualitative screening* of individual hazards, which is mainly based on information available for the site being analyzed and from relevant operating experience, provides a list of sitespecific remaining individual hazards, which cannot be physically excluded. Some hazards can be easily screened out from further analysis because of the general conditions not being met at the site, such as hurricane or tropical cyclone, which do only occur in areas with tropic or sub-tropic climate, or sandstorms, which cannot be assumed based on results of detailed analyses (e.g., for siting and design of building structures) being available for the site ground. In case of a plant site on rock, several hazards such as sinkholes can be easily screened out.

**Figure 1.** Overview of the stepwise approach for screening of hazards and hazard combinations, from [1].

As a result, the following individual hazards remain for the site being investigated after

• Meteorological hazards: C1, C2b, C3b, C4b, C5a, C10, C11, C12, C14, C15, C16, C19, C22,

For these, individual natural hazards remaining after the qualitative screening, the second, quantitative screening step needs to be carried out.
