A modeling study on the oceanic dispersion and sedimentation of radionuclides off the coast of Fukushima

We developed a three-dimensional prognostic oceanic dispersion model that accounted for the phase transfer of radionuclides between seawater, suspended particles, and seabed sediments with multiscale grain sizes. A detailed hindcast of Cs-137 in the seabed sediment off the Fukushima coast was conducted to investigate the transfer mechanism of dissolved Cs-137 derived from the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident toward the seabed sediment. Extensive model-data comparison demonstrated that the model could satisfactorily reproduce the oceanic structure and Cs-137 concentrations in the seawater and seabed sediment. The model successfully reproduced the major features of the observed spatial variation of the Cs-137 activities in the sediment, which represented more than 90% of the sedimentary radiocesium existing in the coastal area off Fukushima several months after the accident. Shear stress associated with the resuspension of the seabed sediment was induced by waves near the shore and by current velocity offshore of the study area. The adsorption of Cs-137 on the seabed sediment differed depending on the particle size, with adsorption on clay being the most substantial. The distribution of Cs-137 in the sediment off the Fukushima coast was formed mainly owing to adsorption from the dissolved phase by June 2011, when the impact of the direct oceanic Cs-137 release from FNPP1 was remarkable. After June 2011, seabed sediment became a source of Cs-137 released to the seawater owing to resuspension with and desorption from the sediment.

Automated summary

A three-dimensional prognostic oceanic dispersion model was developed to study the transfer mechanism of Cs-137 from the Fukushima Daiichi Nuclear Power Plant accident to seabed sediment, with a focus on phase transfer of radionuclides. The extensive model-data comparison showed that the model successfully reproduced the distribution of Cs-137 in seawater and sediment. Adsorption of Cs-137 on seabed sediment varied based on particle size, with significant adsorption on clay, impacting the distribution dynamics of Cs-137 in the sediment off the Fukushima coast.