Three-dimensional modelling of the tidal dispersion of radionuclides in the Irish Sea

Kobayashi, Takuya ; Otosaka, Shigeyoshi  ; Togawa, Orihiko 

A spent nuclear fuel reprocessing plant would release artificial radionuclides from a discharge pipe to the marine environment. These readionuclides spread by advection and diffusion of ocean currents, but their migration processes are different due to physical and chemical forms of radionuclides. We have developed a numerical simulation model system that consists of an ocean current model and a particle random-walk model to describe the radionuclide migration behavior in a shallow water region. The ocean current model is the modified Princeton Ocean Model (POM). The POM calculates tidal currents by giving tide levels at the open boundary. The particle random-walk model, SEA-GEARN, calculates the radionuclides migration in the ocean. Radionuclides that exist in the ocean are modeled in three phases, such as dissolved in seawater, adsorbed with suspended solids and adsorbed with active bottom sediment. The adsorption and desorption processes between the dissolved and solid phases are solved with the kinetic transfer coefficients by the stochastic method. Deposition of suspended solids and resuspension of bottom sediment are also considered. The system has been applied to simulate the long-term dispersion of Cs and Pu in the Irish Sea where these radionuclides are actually released from a spent nuclear fuel reprocessing plant. The calculation results and observations have been compared at several points in order to validate the developed models.