Three-dimensional modelling of the tidal dispersion of non-conservative radionuclides in the marine environment. Application to 239,240Pu dispersion in the eastern Irish Sea

A three-dimensional model to simulate the tidal dispersion of non-conservative radionuclides in a marine environment has been developed. The model solves the three-dimensional hydrodynamic equations using normalized σ coordinates in the vertical and a flow dependent formulation for the eddy viscosity. Simultaneously, the suspended matter equation is also solved in σ coordinates, which includes advective–diffusive transport of particles plus settling. Erosion and deposition are incorporated into the sea bed boundary condition of the equation. It is considered that radionuclides can be present in three phases: solution, suspended matter particles and bottom sediments. Three equations have been formulated, whose solutions give the time evolution of specific activity into each phase. The exchanges of radionuclides between the liquid and solid phase (suspended matter and sediments) have been described in terms of kinetic transfer coefficients, so that the model can be applied under non-equilibrium conditions. The model has been applied to simulate the dispersion of 239,240Pu in the eastern Irish Sea, which is released from Sellafield nuclear fuel reprocessing plant. Computed and measured suspended matter concentrations have been compared for several points in the sea to test the suspended matter sub-model. Also, computed and observed 239,240Pu distributions in water, suspended matter and sediments have been compared for several years. kd distribution coefficients have been computed as well. Model results are, in general, in good agreement with observations.