Treatment of Logical Loops in PSA for Risk Based Decision Making

Abstract

In most PSAs implementing the fault/event tree approach, there are several limitations to get the proper minimal cut sets. One such limitation is the logical loop, which occurs due to mutual dependencies between support systems. Until now, the logical loop has been removed by ignoring some relations between the systems. However, this conventional method might generate some nonsense cut sets or miss some important cut sets since the real relations between the systems are ignored. This kind of error can lead to misinterpretation of PSA results and, therefore, improper decisions. An analytical method to solve the logical loop exactly has been developed by the Korea Atomic Energy Research Institute (KAERI) and implemented in a cut set generation code called the KAERI Reliability Analysis Package (KIRAP). In this method, the Boolean logic of the fault tree is regarded as a kind of network in which the events/gates are regarded as the nodes. The paths of this network are examined by a top-down approach, and the path, which causes the logical loop, is identified and deleted based on the Boolean criteria. The logic of the fault tree is therefore changed into new logic without logical loops while maintaining proper relations between systems.