Modelling the dispersion of radionuclides by a river plume: Application to the Rhone river

A numerical model to simulate the transport of radionuclides in a river plume has been developed. The model solves the hydrodynamic equations, including baroclinic terms and a 1-equation turbulence model, suspended matter equations, including several particle sizes and deposition and erosion of sediments, and the radionuclide dispersion equations. The exchanges of radionuclides between water, suspended matter and bed sediments are described in terms of kinetic transfer coefficients. The dependence of these coefficients with water salinity is explicitly included in the model since in a river plume it changes from freshwater to seawater values. The model has been applied to the Rhone River plume, where radionuclides have been mainly discharged from a nuclear fuel reprocessing plant. Four particle sizes have been considered. Computed currents, salinity patterns, suspended sediment distributions and sedimentation rates are, in general, in agreement with observations. The model has been applied to simulate the transport of 125Sb, 137Cs and 239,240Pu in the river plume. The model provides realistic radionuclide distributions in water, suspended matter and bottom sediments. Other valuable information, as fractions of radionuclides in suspended particles and distribution coefficients, can also be provided by the model.