Example of PWR Severe Accident Sensitivity and Uncertainty Analyses Using MELCOR Code

This paper presents an uncertainty analysis of the source term (ST) in a pressurized water reactor (PWR) station blackout (SBO) accident using the MELCOR2.2 code coupled with DAKOTA via SNAP. Severe accident (SA) simulations in nuclear reactors are fraught with uncertainties due to complex phenomenology and numerical model simplifications. While the best-estimate plus uncertainty (BEPU) methodology is well established in thermohydraulic studies, its application to SA scenarios is relatively new. This study is part of the European Management and Uncertainties of Severe Accidents (MUSA) project, which to some extent, aims to integrate BEPU methodologies into SA simulations. Two figures of merit focused on ST retention in the steam generator (SG) and releases of selected radionuclides to the environment were analyzed. The uncertain parameters used in this study, chosen based on a literature study and project discussions, are related to core degradation and aerosol phenomena. The results highlight significant uncertainties in the ST calculations influenced by variations in the aerosol behavior and core degradation parameters. In conclusion, this study illustrates the application of BEPU methodology to SA uncertainty analysis in a PWR SBO plus SG tube rapture scenario. The findings indicate a potential for improving SA models and reducing uncertainties, contributing to the MUSA project’s goals of enhancing nuclear plant safety analyses and reliability.