Numerical modeling and parameters analysis of marine radionuclide dispersion under the Fukushima Daiichi nuclear accident

Radioactive contaminated water was released into the Pacific Ocean under the Fukushima Daiichi nuclear accident in 2011. Knowledge of the marine radionuclide dispersion behaviors is significant to accurately evaluate radiation effects due to ocean discharge of radioactive contaminated water. The marine radionuclide dispersion is affected by wind field, tide, seafloor topography, temperature and salinity, etc. Significant uncertainty exists in marine radionuclide dispersion simulation with the consideration of complex environmental factors. MIKE 3 FM computational platform was used to establish a Fukushima marine radionuclide dispersion model driven by ocean current and tidal coupling boundary under consideration of various environmental factors. Further, the model accuracy validation with measurement data and inter-models’ comparison for bias mechanism analysis were performed based on the benchmark under the frame of IAEA MODARIA Programme. The effects of vertical distribution of layers mesh, grid resolution, temperature and salinity, horizontal diffusion parameters, wind characteristics as well as tidal forcing on the dispersion behavior were discussed. The results show that the established model can accurately describe the oceanic currents in the coastal waters of Fukushima and predict the dispersion trajectory, as well as concentration level and distribution. More vertical layers, wind-driven forces data and refined mesh for the area of concern should be considered to improve the predictive accuracy of model. The modeling and parameters analysis can help to decrease uncertainty in predicting the levels of radioactivity in the adjutant waters east of the Fukushima site.