83 POSEIDON BOX MODEL DESCRIPTION The compartment model POSEIDON was developed and improved in several papers [II-1–3]. It is a model for radiological assessments of routine and accidental radioactivity releases into the sea and the software is based on a box modelling approach designed to cope with the need for large space and timescale calculations. The dispersion of radionuclides takes place via neighbouring boxes and across the vertical water column where the following dispersion mechanisms are considered: ? horizontal and vertical water exchanges from between boxes; ? adsorption on suspended sediments; ? depletion of activity in suspended materials in equilibrium with the water phase activity; ? exchange of radionuclides between water column and bottom through the molecular di?usion and bioturbation phenomena. A more detailed representation of the water column and its sediment layers, as well as its interaction with neighbouring volumes, is shown in Figure II-1. The boxes describing the water column containing suspended matter are subdivided into a number of vertical layers. The radionuclide concentration for each water column layer is governed by a set of di?erential equations. These equations consider temporal variations in the nuclide concentration, the exchange with adjacent boxes due to advection, sediment settling and turbulent di?usion processes. Furthermore, the transfer of activity from suspended to bottom sediment due to suspended sediment settlement, radioactive sources and radioactive decay is considered. Temporal variations in the three sediment layers located under the water column are described by another set of equations. These equations consider the transfer of radioactivity between water column and sediment, and radioactive decay. The transfer of radioactivity from the upper sediment layer to the water column is described by di?usion in the interstitial water and by bioturbation. Radioactivity in the upper sediment layer migrates downwards by di?usion and by burial at a rate taken as the same at which particles settle from the overlying water. The upward transfer of radioactivity from the middle sediment layer to the top sediment layer occurs only by di?usion. Burial causes an e?ective loss of radioactivity from the middle to the deep sediment layers, from which no upward migration occurs. II-1. TRANSPORT OF RADIONUCLIDES IN WATER AND SEDIMENTS The mechanisms of radionuclide transfer are as follows: activity entering the water column is transported by currents and turbulent di?usion and lost to bottom sediments through sorption on suspended particles which then settle out. The exchange of activity between the upper layer of the sediment and the water column is described as di?usion and bioturbation (modelled as a di?usion process). Activity in the upper sediment layer may di?use downward but there is also an e?ective downward transfer via the continued sedimentation at the top of the sediment layers. Return of activity from the middle sediment to the top sediment occurs only through di?usion. The e?ective loss of activity from middle sediment to deep sediment arises from the continued deposition of sediment.