94 DESCRIPTION OF THE SELFE/IMMSP/KIOST MODEL IV-1. INTRODUCTION The model developed by IMMSP/KIOST for radionuclide transport is based on the finite element hydrodynamic model SELFE [IV-1, IV-2]. The radionuclide transport model describes the key exchange processes shown in Figure IV-1. In the water column, radionuclides in dissolved and particulate phases are transported by currents (advection processes) with the simultaneous in?uence of the turbulent di?usion processes. The radionuclides in the dissolved phase interact with the particulate phase (suspended sediments and bottom deposits). The transfer of activity between the dissolved and particulate phases is described by adsorption– desorption processes. The settling of contaminated suspended sediments and the bottom erosion are important pathways of radionuclide exchanges between bottom and suspended sediments. The transfer of activity between the water column and the pore water in the upper layer of the bottom sediment is governed by di?usion processes. For a correct simulation of radionuclide dispersion 3-D current fields, suspended sediment concentrations, sediment ?uxes of bottom erosion and deposition, as well as composition of bed sediments should be known. In order to carry out the required simulations a set of models, including a 3-D hydrodynamic model, 3-D sediment transport model and the radioactivity transport model were set up. The Eulerian and Lagrangian versions of radioactivity transport models are used. FIG. IV-1. Main processes affecting the radionuclide transport in marine environment.