Abstract
Severe accidents at nuclear power plants may release radionuclides whose atmospheric dispersion transcends national borders. Many of Europe's reactors were commissioned between the late 1970s and the mid-1980s and now operate well beyond their original design life, often under licences extended into the 2030s-2040s. Taking the Almaraz Nuclear Power Plant (NPP) in western Spain - one of at least 17 ageing European reactor sites aged 40-57 years - as a representative case, this study assesses the European-scale atmospheric exposure to a hypothetical severe release of Fukushima-analogue magnitude under current and projected climate conditions. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to compute 48 h forward trajectories and three-dimensional 137Cs dispersion plumes, driven by ERA5 reanalysis (2014-2024) and by 19 CMIP6 SSP5-8.5 simulations bias-corrected with the MBCn multivariate procedure for 2041-2070, using the 137Cs emission inventory of Source Term 5 (JAEA-Katata-6h). Under historical forcing, the dispersion is organised into three recurrent transport modes - westward or near-source (towards Portugal and the eastern Atlantic), eastward (towards the Iberian interior and the western Mediterranean) and northward to north-eastward (towards France) - with opposite seasonal cycles: the westward mode dominates autumn and winter, whereas the continental modes peak in summer. The westward and near-source clusters are also the slowest and shallowest, favouring boundary-layer transport and near-source deposition, so the highest concentrations occur in compact, slow-moving plumes, whereas the fast long-range clusters produce the broadest and most dilute ones. Under SSP5-8.5 the three modes persist, but the winter transport loses its long-range Atlantic reach and concentrates over Portugal and the western Iberian Peninsula, while the summer transport re-orients towards the western Mediterranean as the eastward mode strengthens and the northward mode weakens. Future peak concentrations increase in winter and autumn, decrease in spring and diverge in summer, with higher peaks systematically paired with more compact, near-source plumes. These seasonally and quantile-dependent shifts indicate that historically-informed emergency-preparedness planning may misjudge future transboundary exposure at Almaraz, warranting periodic re-assessment under updated climate projections. The seasonal patterns themselves are site-specific, but the assessment framework is directly transferable to the other long-operating European reactors that will remain in service for decades.