Modelling dispersion processes of radioactive materials in a maritime environment for emergency response to maritime transport accidents involving radioactive material packages

A numerical simulation model that consists of an ocean flow model, called MASs-CONsistent flow simulation model (MASCON), and a dispersion model has been developed to describe the behavior of dispersion processes of radioactive materials in a shallow water region. The use of the model enables several assessment of radioactive materials concentration distributions in the ocean due to release from the sunken package. Further, the dispersion calculation is able to perform in a short time which responds to emergency action for maritime transport accidents involving radioactive materials. Radioactive materials in the ocean are modeled in three phases; a dissolved phase in seawater, a suspended matter phase, and active bottom sediment phase. The adsorption and desorption processes between the dissolved, suspended matter, and active bottom sediment phase are solved by a kinetic approach to describe the transfers of radioactive materials between the liquid and solid phases. The simulation model was applied to simulate the dispersion of 137Cs actually released into the environment by the Fukushima Daiichi Nuclear Power Plant accident in March 2011. A simulated result indicates that the modelling of contribution from all three phases to the seabed sediment is important to reproduce the vertical distribution of radioactive materials in the seabed sediment.