A new method for evaluation of the qualitative importance measures

The importance measures are utilized in the probabilistic safety analysis (PSA) to assess the impact of risk contributors on the selected risk measure. The particular risk measure can be safety system unavailability, the core melt frequency, expected latent fatalities or some other measure of system unreliability or plant risk. The importance measures are divided into quantitative and qualitative. The qualitative importance measures are derived for the qualitative, logic structure of the PSA. The logic structure of the PSA includes the fault tree and event tree models, the failure combinations causing undesired events and the success paths preventing undesired events. The exact logic expression of the selected risk measure is required input for assessment of the qualitative importance measures. The assessment of the exact logic expression is complex even for a small system. A new method of assessment of the qualitative importance of the events in the fault tree is developed and presented in this paper. The qualitative importance of the events is assessed with new qualitative importance measures. The new measures are obtained from the Birnbaum importance measure and application of the min cut upper bound approximation. The minimal cut sets of the analyzed system, as standard qualitative result, are required input for the assessment of the new measures. The developed method is applied on test systems. The ranking of the events based on new qualitative importance measure is compared to the ranking based on structural measure of importance. The structural measure of importance of the events is assessed with application of the binary decision diagram. Obtained results show that ranking of the events based on the new qualitative importance measure is comparable to the ranking obtained by structural importance measure. Utilization of the new qualitative importance measures together with quantitative importance measures for classification of the systems, structures and components is discussed.JRC.G.10-Knowledge for Nuclear Security and Safet