Novel DGT Configurations for the Assessment of Bioavailable Plutonium, Americium, and Uranium in Marine and Freshwater Environments

Time-Integrated Bioavailability Proxy for Actinides in a Contaminated Estuary

Actinides accumulate within aquatic biota in concentrations several orders of magnitude higher than in the seawater [the concentration factor (CF)], presenting an elevated radiological and biotoxicological risk to human consumers. CFs currently vary widely for the same radionuclide and species, which limits the accuracy of the modeled radiation dose to the public through seafood consumption. We propose that CFs will show less dispersion if calculated using a time-integrated measure of the labile (bioavailable) fraction instead of a specific spot sample of bulk water. Herein, we assess recently developed configurations of the diffusive gradients in thin films (DGT) sampling technique to provide a more accurate predictor for the bioaccumulation of uranium, plutonium, and americium within the biota of the Sellafield-impacted Esk Estuary (UK). We complement DGT data with the cross-flow ultrafiltration of bulk seawater to assess the DGT-labile fraction versus the bulk concentration. Sequential elution of Fucus vesiculosis reveals preferential internalization and strong intracellular binding of less particle-reactive uranium. We find significant variations between CF values in biota calculated using a spot sample versus using DGT, which suggest an underestimation of the CF by spot sampling in some cases. We therefore recommend a revision of CF values using time-integrated bioavailability proxies.

Bioavailable actinide fluxes to the Irish Sea from Sellafield-labelled sediments

Nuclear discharges to the oceans have given rise to significant accumulations of radionuclides in sediments which can later remobilise back into the water column. A continuing supply of radionuclides to aquatic organisms and the human food chain can therefore exist, despite the absence of ongoing nuclear discharges. Radionuclide remobilisation from sediment is consequently a critical component of the modelled radiation dose to the public. However, radionuclide remobilisation fluxes from contaminated marine sediments have never been quantitatively determined in-situ to provide a valid assessment of the issue. Here, we combine recent advances in the Diffusive Gradients in Thin Films (DGT) sampling technique with ultrasensitive measurement by accelerator mass spectrometry (AMS) to calculate the remobilisation fluxes of plutonium, americium and uranium isotopes from the Esk Estuary sediments (UK), which have accumulated historic discharges from the Sellafield nuclear reprocessing facility. Isotopic evidence indicates the local biota are accumulating remobilised plutonium and demonstrates the DGT technique as a valid bioavailability proxy, which more accurately reflects the elemental fractionation of the actinides in the biota than traditional bulk water sampling. These results provide a fundamental evaluation of the re-incorporation of bioavailable actinides into the biosphere from sediment reservoirs. We therefore anticipate this work will provide a tool and point of reference to improve radiation dose modelling and contribute insight for other environmental projects, such as the near-surface and deep disposal of nuclear waste.

Passive Sampling Tool for Actinides in Spent Nuclear Fuel Pools

Spent nuclear fuel must be carefully managed to prevent pollution of the environment with radionuclides. Within the framework of correct radioactive waste management, spent fuel rods are stored in cooling pools to allow short-lived fission products to decay. If fuel rods leak, they liberate radionuclides into the cooling water; therefore, it is essential to determine radionuclide concentrations in the pool water for monitoring purposes and to plan the decommissioning process. In this work, we present, to our knowledge, the first passive sampling technique for measures of actinides in spent nuclear fuel pools, based on recently developed diffusive gradients in thin-film (DGT) configurations. These samplers eliminate the need to retrieve and handle large samples of fuel pool water for radiochemical processing by immobilizing their targeted radionuclides in situ on the solid phase within the sampler. This is additionally the first application of the DGT technique for Cm measure. Herein, we make the calibrated effective diffusion coefficients of U, Pu, Am, and Cm in borated spent fuel pool water available. We tested these samplers in the fuel pool of a nuclear facility and measured samples using accelerator mass spectrometry to provide high-precision isotopic reports, allowing for the first independent implementation of a recently developed technique for dating nuclear fuel based on its Cm isotope signature.