Enhancements to the accident precursor methodology

A feasibility study for developing an improved tool and improved models for performing event assessments is described. The study indicates that the IRRAS code should become the base tool for performing event assessments, but that modifications would be needed to make it more suitable for routine use. Alternative system modeling approaches are explored and an approach is recommended that is based on improved train-level models. These models are demonstrated for Grand Gulf and Sequoyah. The insights that can be gained from importance measures are also demonstrated. The feasibility of using Individual Plant Examination (IPE) submittals as the basis for train-level models for precursor studies was also examined. The level of reported detail was found to vary widely, but in general, the submittals did not provide sufficient information to fully define the model. The feasibility of developing an industry risk profile from precursor results and of trending precursor results for individual plants were considered. The data sparsity would need to be considered when using the results from these types of evaluations, and because of the extremely sparse data for individual plants we found that trending evaluations for groups of plants would be more meaningful than trending evaluations for individual plants