Mathematical Modeling of Radioactive Contaminants in the Fukushima Environment

Significant amounts of radioactive materials were released to the atmosphere from the Fukushima Daiichi nuclear power plant after the accident caused by the major earthquake and devastating tsunami on March 11, 2011. Accurate and efficient prediction of the distribution and fate of radioactive materials eventually deposited at the surface in the Fukushima area is of primary importance. In order to make such a prediction, it is important to gather information regarding the main migration pathways for radioactive materials in the environment and the time dependences of radioactive material transport over the long term. The radionuclide of most concern in the Fukushima case is radioactive cesium. Previous surveys indicate that the primary transportation mechanisms of cesium are either soil erosion and water transport of sediment-sorbed contaminants or transport of dissolved cesium in the water drainage system such as by rivers. A number of mathematical models of radioactive contaminants, with particular attention paid to radiocesium, on the land and in rivers, reservoirs, and estuaries in the Fukushima area are developed. Simulation results are examined while simultaneously implementing field investigations. For example, the orders of magnitude of the radiocesium concentration on the flood plain of the Ukedo River by model prediction and field investigation results were both 10⁵ Bq/kg. Microscopic studies of the adsorption/desorption mechanism of cesium and soils have been performed to shed light on the mechanisms of macroscopic diffusive transport of radiocesium through soil. The maximum exchange energy between cesium and prelocated potassium in the frayed edge site was simulated to be 27 kJ/mol, which reproduces the corresponding value previously achieved by experiments. These predictions will be utilized for assessment of dose from the environmental contamination and proposed countermeasures to limit dispersion of the contaminants.