Initiating Events and Scenarios

Instead of analyzing the engineered systems of the plant and the potential event sequences in detail (vid. Chap. 9) generic values for the occurrence of accidents are used. These depend on numerous factors, amongst them 

All of these and further important factors of influence are not yet known at the moment of land-use planning. That is why the recourse is to base considerations on a flctitious generic plant. This is difficult because amongst others reliability data at the level of process units are available, if at all, only for some classes of units, e.g. 

That is why the following approach is chosen. The information of the data bank of the Federal Institute for the Environment ZEMA enables one to derive an expected value of 6.4 x 10 per plant and year for an accident with harm to persons outside the premises of plant. This value is arrived at using Bayesian zero failure statistics (vid. Example 9.6). It is about one order of magnimde smaller than that for a fatal occupational accident in the chemical industry, for which about 2.2 X 10 a is obtained (vid. Eig. 1.1). 

In order to put these frequencies into perspective they can be compared with the frequency of death by lightning. This is stated to be 10 a [8] and is considered as negligible. 

Assuming that about 10 % of the accidents cause harm to people we would expect a frequency of a major release of about 6 x 10 (6.4 X 10 /0.1 6 x 10 ) per plant and year. 

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