84 FIG. II-1. Schematic diagram of the activity transfer processes in POSEIDON model. POSEIDON equations are obtained by averaging the 3-D transport equation for the dissolved radionuclide concentration Cw (Bq/m3) and the concentration in the three layers of the bottom sediment. It is assumed that the activity in the water column is partitioned between the water phase and the suspended sediment material, using the following relation: ?? = ???? (II-1) where Cs (Bq/kg) is radioactivity concentration adsorbed in suspended sediment, kd is the radionuclide distribution coe?cient (m3/kg). The equation for the water column layers read as follows: ???,? ?? =? ???,? ??,? ??,? ? ??,? ??,? ??,?? ? + ??,???(?,?,???) ? ???,? + ????,? + ??,? ?? ???? + ??,? (II-2) For the top sediment layer: ??? ?? = ?(?? + ?? + ?)?? + ?? ??,? ??,???,? + ??,? ??,? ???? (II-3) and for the middle sediment layer: ??? ?? = ?(?? + ?? + ?)?? + ??,? ??,? ???? (II-4) where Fi,j is the water ?ux from box i to box j; V0 is the box volume; hi is the depth of the water compartment; Lt and Lm are the depth of top and middle sediment layers respectively; Q is the point source of the activity; ?1 ... ?5 are the transfer coe?cients, whose values depend on the characteristics of the radionuclide and sediments.