Importance measures for use in PRAs and risk management

There are many quantities estimated in probabilistic risk assessments (PRAs) to index the level of plant safety. If the PRA is to be used as a risk management tool to assist in the safe operation of the plant, it is essential that those elements of the plant design and its mode of operation that have the greatest impact on plant safety be identified. These elements may be identified by performing importance calculations. There are certain decisions that must be made before the importance calculation is carried out. The first is the definition of the events for which importance is to be evaluated; that is, to what level of resolution the analysis is to be performed. The second decision that must be made--and the major subject of this paper--is the choice of importance measure. Many measures of importance have been proposed; this discussion is restricted to three: the risk achievement (or degradation) worth, the risk reduction worth, and criticality importance. In the paper these measures of importance are defined, their interrelationships are discussed, and a generalized importance measure is introduced. The use of these three measures is compared and their advantages and disadvantages are discussed