An Integrated Watershed Modeling to Assess the Long-term Fate of Fukushima-Derived Radionuclides

The Fukushima Dai-Ichi Nuclear Power Plant (NPP) accident was induced by the Great East Japan Earthquake and the subsequent Tsunami on March 11 th , 2011. Radionuclides (RNs) which emitted into the atmosphere from the NPP fell onto the land surface by precipitation. The fallout RNs were then redistributed through surface/subsurface water flow and sediment transport. To study this redistribution phenomena, we developed a novel integrated watershed modeling tool to assess the long-term fate and transport of the fallout RNs and their environmental impact. The feature of the simulator is to compute fully-coupled behavior of RNs transport among channels, overland and subsurface using physics-based 3D numerical model. An actual reservoir basin was selected as a study area and the measured concentration was reproduced to obtain a better understanding of the radiocesium behavior. Reasonable matching between the simulated 137 Cs concentration and the observed one was achieved. The computed results showed that suspended sediment was the dominant transport media of 137 Cs, and its fraction of the total 137 Cs flux was more than 95%. Furthermore, it was indicated that the total discharge of 137 Cs by surface water flow and sediment transport was less than 5% of the initial inventory, and most of the deposited 137 Cs was still remaining within the watershed. These quantitative results and visualized contents using GIS shall contribute to effective discussion on planning of regional decontamination and reconstruction.