01 November 2002
Event trees
The human reliability analysis (HRA) event tree is a visual tool to help control or process engineers; health, safety, and environmental specialists; or whoever is doing safety integrity level selection at the plant relate all the events in a task.
In low-level scenarios, the operator must perform different steps properly. Unexplained events might occur that he might miss the first time but realize later. The HRA event tree shows corrected mistakes - the logic and thought process on when you can recover from mistakes.
In an event tree (as presented in the THERP Method-NUREG/CR-21278), each performed task (represented by a small filled circle) has two paths: success and failure. Success usually appears as a diagonal line down and to the left, while a failure line goes down and to the right. A description of the task that includes the task letter appears for each path in the tree. The success path shows the lowercase letter of the task, while the failure path shows the uppercase task letter.
Recovery actions, and sometimes success-to-failure transitions, appear as dashed lines. Regardless of transition type, the arrow of the dashed line shows the correct logic. Some analyses supplement the THERP event tree method to include an open circle representing successful completion of a task set and a filled triangle representing the termination of a failure path.
|
Typical HRA event tree
|
|
Simplified HRA event tree
|
|
InTech Home