Fault tree analysis is a useful categorization tool with which the interrela­tionships between reactor components, their failures, and the reactor safety features can be defined. It provides a means for ensuring that safety analysis is all-inclusive, and it provides eventual potential for quantifying accident probabilities.

1.6.1 Definitions

A fault tree is a sequence of events which leads from one or more faults to the causes of those faults.

Systems analysts use such fault trees to: (a) define critical paths in the accident analysis; (b) calculate the probabilities of failures leading to given consequences or of consequences occurring in the system from one of a number of different initiating faults; and (c) specify safeguards against damaging consequences for each branch of the tree.

For different purposes the different trees available that will be discussed are: (a) a single-failure tree defined as a successive analysis of the causes of a single undesirable event; (b) a multiple-failure tree defined as an analysis of the consequences of a whole range of faults leading to a whole range of possible safe and unsafe terminations; and (c) an accident-process tree defined as a successive analysis of the consequences of a single fault. (This tree is a single branch of the multiple-failure tree and the reverse of the single-failure tree.)