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Probabilistic risk assessment (PRA) is a method used to identify and evaluate the risk of terrorist attacks in complex technological security systems in order to improve their safety and performance. PRA requires determining the probabilities and consequences of a number of potential scenarios. In this paper, a dynamic generation of event tree to model different hypothetical attack scenarios is proposed. An innovative approach is described to limit the size of this tree, which employs specific truncation rules for retaining only those scenarios with risk estimates above a certain threshold. The proposed method which takes into account the consequences and the probabilities of attack scenarios allows low probability high risk attack to be considered.