117 DESCRIPTION OF THE JAEA: SEA-GEARN MODEL The SEA-GEARN model [IX-1] is a particle random-walk model which has been used to simulate radionuclide transport in the Pacific Ocean. SEA-GEARN uses 3-D velocity data calculated by an ocean general circulation model as the input variables. If non-conservative radionuclides are concerned, the interactions with particulate matter need to be considered. In order to take these situations into account, a new model that solves three-phase interaction [IX-2], was adopted in SEA-GEARN. Radionuclides are assumed to exist in three phases, such as dissolved, large particulate matter (LPM) and active bottom sediment, respectively. This model has the following assumptions: (i) the LPM is an aggregate which has a single radius and density; and (ii) the movement of each particle is governed by velocity, di?usivity and settling velocity of the particle itself. Moreover, the following assumptions are also made: (i) the dissolved phase consists of radionuclides that are dissolved and adsorbed onto fine (diameter < 0.8 µm) particles without settling velocity; (ii) the LPM phase consists of radionuclides that are adsorbed on settling suspended particles; (iii) the active bottom sediment phase consists of radionuclides which are adsorbed on the LPM phase and deposited on the seabed. These particles may resuspend according to the bottom water velocity. Kinetic transfer coe?cients are used for the calculation of adsorption/desorption between dissolved phase and LPM phase or between dissolved phase and bed sediment phase [IX-3]. The radionuclides migration model for the dissolved phase is written as: ?(??) ?? + ?(???) ?? + ?(???) ?? + ?(???) ?? = ? ?? ??? ??? ?? ? + ? ?? ??? ??? ?? ? + ? ?? ??? ??? ?? ? ? ????? + ?????? ? ??? + ?? (IX-1) where Cd is the dissolved phase radionuclide concentration, u, v and w are the 3-D current velocity components and Kh and Kz are the horizontal and vertical di?usion coe?cients, respectively. The fourth term of the right hand side is the adsorption from dissolved phase to the LPM phase, the fifth term the desorption from the LPM phase to dissolved phase, k1m and k?1 are the kinetic transfer coe?cient for the LPM, Cs is the LPM phase radionuclide concentration, m is the LPM concentration, ? is the decay constant and Pd is the input of dissolved radionuclides from the source point. The surface boundary condition is: ??? ? ?? ??? ?? = 0 (IX-2) The sea bottom boundary condition is: ?? ??? ?? = ????? ? ??? ??????(???) ? (IX-3) where k1s is the kinetic transfer coe?cient for the active bottom sediment phase, As is the active bottom sediment phase radionuclide concentration, L is the mean mixing depth of the active bottom sediment, ?s is the bulk density of the active bottom sediment, f is the fraction of active bottom sediments, p is the porosity and ? the thickness of the water layer which interacts with the active bottom sediment phase.