Application of Expanded Event Trees combined with uncertainty analysis methodologies

Highlights: • Expanded Event Trees of the sequence with uncertainties are quantified. • Extended BEPU using parametric, non-parametric statistics. • EET branches contributing to damage are successful sequences. • Different interpretations of the results, including probabilistic and deterministic methods are presented. During the last decades, the safety analysis of nuclear power plants, have been shifting from conservative models and hypotheses to Best Estimate. Within this trend, the different safety analyses can be categorized depending on their associated conservatisms. The approach that includes Expanded Event Trees together with Best Estimate model and conditions plus uncertainty avoiding conservatisms in system availability can be referred as Extended BEPU (EBEPU). In this sense, an Extended BEPU safety analysis relies on Expanded Event Trees and Best Estimate models to study an accidental sequence. In the present paper, an EBEPU study is performed, using a loss of coolant accident in a pressurized water reactor modeled with the TRACE code. A 5% core power uprate is used as an example of safety margin EBEPU analysis. The paper presents parametric and non-parametric uncertainty approaches. Observing the results from all event tree sequences, it is seen that the branches that contribute the most to the Core Damage frequency are successful branches, with all safety systems available, and some Core Damage branches have a possibility of success.