Risk significance and safety significance

Existing measures of the risk significance of elements of risk models (such as the Fussell–Vesely, or ‘F–V’, importance of basic events) are based on the properties of cut sets containing the element. A measure of safety significance (prevention worth, or PW) is proposed, based on the properties of path sets containing the element. A high value of F–V means that cut sets containing the element contribute significantly to top event frequency; a high value of PW means that path sets containing the element contribute significantly to top event prevention. The properties of PW as a measure of basic event significance are illustrated first with a simple block diagram example, and then with an example based on nuclear power plant risk models. PW can also be understood as a property of a set of success scenarios, and as such, can be applied more broadly than just as a measure of element significance.