87 DESCRIPTION OF THE THREETOX MODEL III-1. INTRODUCTION The modelling system THREETOX was developed for simulating the dispersion of radionuclides and other contaminants at local and regional scales [III-1–3]. The system includes models for hydrodynamics, ice dynamics–thermodynamics and models for sediment and radionuclide transport (see Figure III-1). The prognostic variables of the hydrodynamic model are the three components of the velocity fields, temperature, salinity, water surface elevation and kinetic energy of turbulence and its dissipation rate. The ice model predicts the ice drift, thickness and ice concentration. The interactions between water, ice and atmosphere are parameterized by the bulk aerodynamic formulae, whereas the transport, deposition and resuspension of several fractions of sediments are calculated by the sediment transport model. The radionuclide concentration in solute, suspended sediments and the seabed is predicted by the radionuclide transport model and a one-step reversible model was used to describe the exchanges of radionuclides between water and sediments. III-2. HYDRODYNAMICS Hydrodynamics are simulated on the basis of the 3-D, time dependent, free surface, primitive equation model in the Boussinesq approximation. The model equations are written in curvilinear orthogonal coordinates. The governing Reynolds averaged equations of continuity, horizontal momentum, conservation equations for temperature T and salinity S, state equation and hydrostatic relation can be written in Cartesian coordinates as: ?? = 0 (III-1) ?? ?? + ? ? ?? ? ?? = ? ? ?? ?? ?? + ? ?? ??? ?? ?? ? + ? ?? ??? ?? ?? ? + ? ?? ??? ?? ?? ? (III-2) ?? ?? + ? ? ?? ? ?? = ? ? ?? ?? ?? + ? ?? ??? ?? ?? ? + ? ?? ??? ?? ?? ? + ? ?? ??? ?? ?? ? (III-3) ?? ?? + ? ? ?? = ? ?? ???? ?? ?? ? + ? ?? ??? ?? ?? ? + ? ?? ??? ?? ?? ? + (???) ????? ?? ?? (III-4) ?? ?? +? ? ?? = ? ?? ???? ?? ?? ? + ? ?? ??? ?? ?? ? + ? ?? ??? ?? ?? ? (III-5) ?? = ??(?, ?, ?) (III-6) ?(?, ?, ? , ?) = ?? +???? +? ? ?? ? ? (?, ?, ? ??) ??? (III-7) where t denotes time, U = (u, v, w) is the velocity, ? is surface elevation, f is the Coriolis parameter, pa is the atmosphere pressure, ?w is the water density, ?o is the undisturbed density, Cpw is the water heat capacity and (1 ? A)I is solar insolation absorbed in the water. Here ice concentration (compactivity) A is the fractional area covered by ice, whereas (1 ? A) is the fractional open water [III-4].