Event tree analysis

Event tree analysis (ETA) is a forward, top-down, logical modeling technique for both success and failure that explores responses through a single initiating event and lays a path for assessing probabilities of the outcomes and overall system analysis.

[1] This analysis technique is used to analyze the effects of functioning or failed systems given that an event has occurred.

The UKAEA study used the assumption that protective systems either worked or failed, with the probability of failure per demand being calculated using fault trees or similar analysis methods.

The conclusions of the US EPA study of the Draft WASH-1400[3] acknowledges the role of Ref 1 and its criticism of the Maximum Credible Accident approach used by AEC.

[4] Performing a probabilistic risk assessment starts with a set of initiating events that change the state or configuration of the system.

The initiating event starts at the left side as a horizontal line that branches vertically.

When the event tree diagram has reached the end state for all pathways the outcome probability equation is written.

Event Tree Analysis makes it easy to see what pathway creating the biggest probability of failure for a specific system.

Though ETA can be relatively simple, software can be used for more complex systems to build the diagram and perform calculations more quickly with reduction of human errors in the process.

SCRAM is an example open-source tool that implements the Open-PSA Model Exchange Format open standard for probabilistic safety assessment applications.