A case study of radionuclide physicochemical characterization and dispersion modelling at the European Spallation Source under a postulated worst-case scenario

This study characterizes the physicochemical properties and evaluates the atmospheric dispersion of radionuclides potentially released from the European Spallation Source (ESS) under a postulated worst-case loss-of-coolant accident scenario. The primary objective was to quantify near-surface air concentrations and deposition. Likely chemical forms were identified through a literature-based evaluation of accident progression, leading to the assumption that released species form independent oxide particles with a reference geometric mean diameter of 300 nm; alternative diameters of 200, 500, 750, and 1000 nm were also considered. Atmospheric transport was simulated using FLEXPART (version 11), driven by high-resolution meteorological data for 2021, representative of historical weather conditions. Simulations were performed independently for non-radioactive tungsten oxides and selected radionuclide oxides that contribute the most to the effective dose during the first seven days post-accident. Results were evaluated for urban areas: Östra Odarslöv, Lund, Stångby, and Södra Sandby. Integrated annual air concentrations were highest at the ESS and Östra Odarslöv. Dry deposition dominated particle removal at all locations. Particle size strongly influenced dispersion, with a relative minimum in dry deposition around 500 nm; at sites outside the prevailing wind direction, air concentrations varied little between 500 and 750 nm, and wet deposition showed limited variation around 300 and 750 nm. Derived radionuclide-to-tungsten oxide deposition ratios provide insight into key radionuclides for rapid environmental assessment. Overall, the results quantify potential exposure-relevant metrics and support emergency preparedness and radiological consequence assessment, while highlighting the need for experimental validation of assumed chemical forms and aerosol characteristics.