Logical Loop Breaking Method in Fault Tree Analysis

Logical loops or circular logics are interpreted as circular supporting relations among systems or their fault trees. The logical loops could be found by examining the supporting relations among the systems before developing their fault trees. The merged fault tree is created by combining the system fault trees. The logical loop in a merged fault tree exists in a shape of a circular connection of gates. There are two ways to break logical loops such as the analytical and manual breaking methods. Guidance for the manual breaking of the logical loops is provided in NUREG/CR-2728. ASME probabilistic risk assessment standard recommends that a significant conservatism or non-conservatism be avoided when breaking the logical loops. Yang presented an analytical method to break the logical loops. By using the analytical method, KIRAP and FTREX recursively search and break the logical loops in a fault tree and then solve the broken fault tree