Event tree analysis

Event tree analysis (ETA) is a forward, bottom up, 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. This analysis technique is used to analyze the effects of functioning or failed systems given that an event has occurred. ETA is a powerful tool that will identify all consequences of a system that have a probability of occurring after an initiating event that can be applied to a wide range of systems including: nuclear power plants, spacecraft, and chemical plants. This Technique may be applied to a system early in the design process to identify potential issues that may arise rather than correcting the issues after they occur. With this forward logic process use of ETA as a tool in risk assessment can help to prevent negative outcomes from occurring by providing a risk assessor with the probability of occurrence. ETA uses a type of modeling technique called event tree, which branches events from one single event using Boolean logic.

The name Event Tree was first introduced during the WASH-1400 nuclear power plant safety study (circa 1974) where the WASH-1400 team needed an alternate method to fault tree analysis due to the fault trees being too large. Though not using the name event tree the UKAEA first introduced ETA in its design offices in 1968. Initially to try to use whole plant risk assessment to optimize the design of a 500MW Steam Generating Heavy Water Reactor. This study showed ETA condensed the analysis into a manageable form. ETA was not initially developed during WASH-1400, this was one of the first cases in which it was thoroughly used. 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. ETA identifies all sequences which follow an initiating event. Many of these sequences can be eliminated from the analysis because their frequency or effect are too small to affect the overall result. A paper presented at a CREST symposium in Munich,Germany in 1971 shows how this was done. The conclusions of the US EPA study of the Draft WASH-1400 acknowledges the role of Ref 1 and its criticism of the Maximum Credible Accident approach used by AEC. MCA sets the reliability target for the containment but those for all other safety systems are set by smaller but more frequent accidents and would be missed by MCA.

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