ITEM TOOLKIT

Chapter 12 Event Tree 257
Once the system events have been defined, they can be combined to derive the various end states or accident scenarios. In
the graphical representation, columns depict the events, and the horizontal lines represent the success, failure or partial
failure branches. Each combination of these branches from left to the right depicts a path or a scenario ending in a particular
end state or consequence.
Once the Event Tree has been constructed, the next step is the quantification of the event probabilities. Each event,
representing a system or a function failure, can be quantified using basic event quantification directly or linked to the top
event (or any other gate) of a Fault Tree. Other probabilistic models such as Predictions, RBD or Markov models can also
be used. Upon evaluation these fault trees (or basic events) would be linked together to derive the conditional probability of
each event (or branch) and the multiplication of these conditional probabilities for each scenario shall give the probability of
occurrence of final consequences or the accident scenarios.
For further risk analysis, each of the end state scenarios can be further investigated for respective cut-sets. Also, scenarios
ending in same or similar end states can be joined together to get a joint probabilistic result. Weighting of the end scenarios
in various categories such as financial, environmental, etc. can help devise a complete category-wise risk analysis.