Forty-Year Time Series of Anthropogenic Uranium in Fucus vesiculosus from Kattegat: Validating ²³⁶U Releases from European Nuclear Reprocessing Plants

An accurate input function of uranium-236 (236U) is essential for its numerous applications in environmental and ocean studies. This work reveals potential overestimation on 236U releases from the Sellafield (SF) nuclear reprocessing plant (RP) by earlier estimations using shells. We report a 40-year time series dataset of uranium isotopes in seaweed seasonally collected in the Kattegat, downstream of radioactive discharges from the European RPs, namely La Hague (LH) and SF. Comparison between our measured 236U concentrations in seaweed and model-simulated values derived from 236U releases of LH and SF suggests that the previously reconstructed SF discharges may have nearly 1 order of magnitude overestimation. Such overestimation would introduce large uncertainties when 236U is used as a point source in environmental tracer applications and ocean model validation. We foresee that this work will significantly improve the applications of radioisotope tracers in oceanic studies such as the spreading of marine pollutants, ocean mixing, and circulation, especially in the regions of the North Atlantic and Arctic Oceans.