Insights from using influence diagrams to analyze precursor events

Precursor analysis, the evaluation of “near misses,” has been an activity of the Nuclear Regulatory Commission for almost twenty years. One item that has remained constant over this time is that the focus of the analysis has been on modeling the scenario using a risk model and then utilizing the results of the analysis to determine the severity of the precursor incident. The investigation of precursor events can be used as a source of information for the construction of a structured methodological approach for operational decisions. This methodology, which is the focus of a research project currently underway, aims at the evaluation of the optimal strategy during a pre-core-damage incident or off-normal situation. The methodology is based on the integration of probabilistic safety assessment (PSA) and decision analysis tools such as influence diagrams/decision trees. We demonstrate this technique via an evaluation of a U.S. precursor incident.