The impact of oceanic circulation and phase transfer on the dispersion of radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant

The mechanism behind the dispersion of radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant on March 2011 is investigated using a numerical model. This model is a Lagrangian particle tracking – ocean circulation coupled model that has the capability of solving the concentration of radionuclides for those dissolved in seawater and those adsorbed in particulates and bottom sediments. Model results show the radionuclides dispersing rapidly to the interior of the North Pacific along the Kuroshio Extension once they enter a meso-scale eddy. However, radionuclides are also found to remain near the coast with their spatial pattern depending strongly on the oceanic circulation during the first month of the release. This is when most of the adsorption to bottom sediments occurs. If the offshore advection were weak during this period, many radionuclides will be adsorbed to bottom sediments and remain on the coast for some time. If vertical mixing is weak, less radionuclide reach the sea floor and get adsorbed to bottom sediments. More radionuclides will then disperse to the open ocean.