4. Describe the sequences of events to determine the nature of the events identified at the output of the tree.

An event sequence is made up of the initiating event and a combination of operation or non-operation of the mitigation barriers.

Strict application of the method's basic principle (i.e. for each barrier, study the case where it "works" and the case where it "doesn't work") leads to 2 ⁿ RHS (with n number of barriers limiting the intensity of the consequences associated with the initiating event). For the tree shown in the figure "Event tree structure", valuing two safety barriers leads to an output of 2 ² , i.e. four secondary dreaded events.

In practice, if several safety barriers fulfill the same safety function, it is not necessary to create the entire tree structure. The event tree shown in figure "Structure of a simplified event tree" is a so-called...