Initial Spread of ¹³⁷Cs over the shelf of Japan: a study using the high-resolution global-coastal nesting ocean model

The 2011 Tohoku earthquake caused radionuclide 137Cs be directly released into the ocean from the Fukushima Dai-ichi nuclear power plants. A high-resolution global-coastal nesting ocean model was established to simulate the initial spread of 137Cs as conservative tracer over the shelf of Japan after the accident. The major advantage in the current model system is to use unstructured grids to resolve the power plant and the coastal geometry with a grid resolution much higher than that used in previous modeling experiments. Therefore, it gives us an opportunity to examine the necessity whether the detailed structure of the Power plant should be considered for numerical experiment of 137Cs dispersion or not. This could provide us an alternative insight into the physical processes that lead to its spread of 137Cs over the shelf of Japan. Our results suggested that to resolve the dispersion process from the source point to the south and north discharging canal is critical for an accurate prediction of the spread of 137Cs to the 30 km sites off the coast. Moreover, a 2 km grid resolution along Japan coast is probably not high enough to resolve the plume correctly. Finally, the model-data comparison suggested that the physical process associated with the transfer of dissolved 137Cs into the sediment phase could potentially be important and should be considered in the future tracer modeling.