PSA Construction

The initiators are separated into two classes those for which the event iree/fauJt tree analysis is appropriate and those for which it is not. The former are called internal initiators and the latter, external initiators (externalities). If dependencies are accounted for by modifying the branching probabilities of the event tree, both internal and external initiators can be accounted for in the same event tree. 

The systems list, across the top of the event tree, specifies the systems that must be analyzed to obtain the branching probabilities of the event tree. For complex reliable systems, fault tree or equivalent analysis may be needed to obtain system probability from component probabilities. For less reliable systems, the branching probability may be obtained from plant records with cautions regarding system interactions. 

Determining which accident sequences lead to which states requires a thorough knowledge of plant and process operations, and previous safety analyses of the plant such as, for nuclear plants, in Chapter 15 of their FSAR. These states do not form a continuum but cluster about specific situations, each with characteristic releases. The maximum number of damage states for a two-branch event trees is 2 where S is the number of systems along the top of the event tree. For example, if there are 10 systems there are 2 = 1,024 end-states. This is true for an unpruned event tree, but. in reality, simpler trees result from nodes being bypassed for physical reasons. An additional simplification results 

The left side connects with the Initialor, the right side with the damage stale the lop defines the operations, branching probabilities ere inserted at the nodes 

A dependency analysis determines which systems depend on other systems. This may require iterating the event tree construction to put a support system before the systems affected. 

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This PSA construction process provides results but tends to obscure the interpretation of what led to these results. Sensitivity analyses are basically studies of the effect on risk due to small or large changes in system/component reliability or operability (see Section 2.8). 

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