Iodine Source Term Uncertainty and Sensitivity Analysis with the Severe Accident Code ASTEC: Focus on Iodine Chemistry, Aerosol Behavior, and Containment Pressurization

During a severe accident (SA) in a nuclear power plant, iodine release to the environment [iodine source term (ST)] is a critical issue due to iodine’s significant contribution to radiological consequences. This study aims to identify the parameters that govern iodine release to the environment in the context of a total station blackout scenario in a French 900-MW(electric) pressurized water reactor. The investigated parameters are related to iodine chemistry, aerosol phenomena, and containment pressurization. An Accident Source Term Evaluation Code (ASTEC) all-modules computation is first conducted to model core degradation and provide boundary conditions for containment calculations. Uncertainty propagation is then applied to the containment calculations, incorporating probabilistic distributions and ranges for key input parameters. Sensitivity analysis is finally conducted using partial rank correlation coefficients to quantify the influence of modeling, plant, and scenario uncertainties. Results indicate that in the short term, the iodine ST is primarily governed by aerosol-related parameters, including particle size, shape factor, density, and initial isotope inventory. Over longer timescales, the release becomes increasingly sensitive to parameters affecting containment venting (e.g., delay in aligning the residual heat removal system, concrete properties), washing efficiency, the iodine gaseous fraction reaching the containment (when its variation range is sufficiently high), and several uncertainties inherent to iodine chemistry models. This work provides enhanced insight into ASTEC behavior and identifies the most influential parameters contributing to iodine ST uncertainty. These findings support future efforts to refine SA models by prioritizing the reduction of key uncertainties to improve predictive capability and safety assessments.