From modelling to management: improving decision-making through contextualized marine radiological impact assessments...

Adapted management of accidental release of radionuclide to the sea relies especially on the application of radiological impact assessment models. While recent site-specific marine transport models provide detailed simulations, their outputs are often presented without considering maritime boundaries, that imply in practice different legal frameworks and may therefore lead to different management strategies within a physically continuous area. This study addressed that gap by integrating maritime boundaries in a radiological impact assessment of a hypothetical accidental liquid release of ¹³⁷Cs at the Flamanville NPP in the English Channel. The assessment combined the STERNE marine dispersion model, which integrates biokinetic equations, and dosimetric calculations based on the ERICA Tool. Impact characterization focuses on the contamination of non-human biota, for their own protection, and relates to benchmarks for radiation protection. Potential seafood activities are assessed against regulatory limits for commercialization after a nuclear accident. One-year dispersion simulations were carried out for a one-week Fukushima-type release, accounting for seasonal fluctuations in hydrodynamics. Spatial dispersion showed an extensive plume crossing territorial seas, notably contaminating the territorial sea of Jersey and Guernsey, two British Crown Dependencies. The plume arrived earlier at Jersey where higher activities are estimated. Calculated activities and dose rates within those territorial seas remained mainly below radiation protection benchmarks. To improve communication and decision-making, a novel indicator was introduced for the impact characterization: the number of days during which benchmarks values are exceeded at each grid cell. These exceedance maps offer a time-based, easily interpretable and transparent output that could enhance risk communication. It provides an adapted tool when considering regulatory uncertainties. For instance, the time perspective could make it easier to forecast economic loss and ensure for the public a better understanding of authority decisions, restoring consumer confidence in marine seafood and enabling economic activity to recover more quickly. In this study, Flamanville vicinity showed up to 70 exceedance days when considering seafood marketing limits, while values were much lower in the other parts of the Channel.