13 Event trees

Event trees identify various sequences of events, both failures and successes that can lead to accidents. It is a forward, bottom up tool, starting with a single initiating event and determining the events leading to a final state. Event tree analysis is generally applicable for almost any type of risk assessment application, but used most effectively to model accidents where multiple safeguards are in place as protective features. Event trees provide systematic coverage of the time sequence of event propagation, either through a series of protective system actions, normal plant functions, operator interventions and incident consequences. They are really one of the first steps in a probabilistic risk assessment. Event trees are a tool that makes it more straightforward to assess what pathway is creating the greatest probability of failure for a specific system.

By 570SJR – Own work, CC BY-SA 3.0

Methodology

1. Define the system: Define what needs to be involved or where to draw the boundaries.
2. Identify the accident scenarios: Perform a system assessment to find hazards or accident scenarios within the system design.
3. Identify the initiating events: Use a hazard analysis to define initiating events.
4. Identify intermediate events: Identify countermeasures associated with the specific scenario.
5. Build the event tree diagram
6. Obtain event failure probabilities: If the failure probability can not be obtained use fault tree analysis to calculate it.
7. Identify the outcome risk: Calculate the overall probability of the event paths and determine the risk.
8. Evaluate the outcome risk: Evaluate the risk of each path and determine its acceptability.
9. Recommend corrective action: If the outcome risk of a path is not acceptable develop design changes that change the risk.
10. Document the entire process on the event tree diagrams and update for new information as needed.

Event trees as part of PRA was essentially invented for nuclear power plants. Of course, they are still used today, and any new reactor design will also apply them as part of the PRA. So, it should be no surprised that the NRC literally wrote the book on fault and event trees.