107 DESCRIPTION OF THE KAERI: LORAS MODEL After the Fukushima Daiichi NPP accident, a large amount of radioactive material was released into the ocean as well as the atmosphere. Therefore, it is necessary to evaluate marine dispersion for radiological emergency preparedness against a nuclear accident. From this perspective, an oceanic dispersion model named LORAS was developed by KAERI in order to evaluate the transport characteristics of the radionuclides released into the sea for a nuclear accident [VI-1]. The model was designed to calculate radionuclide concentrations in seawater, suspended matter and bed sediments in time and space using a particle tracking method. The particle tracking technique has some advantages over finite di?erence methods, in particular, numerical di?usion is not introduced and the exact position of the release point may be specified. Thus, it is not necessary to assume that the discharge is instantaneously mixed into a grid cell of a given size. A passive particle is transported by current components and dispersed by turbulent motion. Currents are supplied by the hydrodynamic circulation model and turbulent dispersion is evaluated using a random walk method [VI-1, VI-2]. The dispersion of reactive and non- reactive radionuclides may also be simulated in the model. 3-D turbulent di?usion and the pollutant interactions between water, suspended matter and bottom sediments are simulated using a stochastic method [VI-3]. The movement of the particle is represented by the sum of the movements due to advection by the current and turbulence. The new position xj of a given particle after a time step ?t is represented as follows: ??(? + ??) = ??(?) + ??(?)?? + ???(?)?? (VI-1) where vj are the oceanic currents in the three spatial directions (j = 1, 2, 3) and ??? are the turbulent motion (j = 1, 2, 3). Three-dimensional turbulent mixing is computed by a random walk method: ??,?? = ?12??,???? (VI-2) ??? = ?2????? (VI-3) where Kj are di?usion coe?cients in each corresponding direction of space and R is a random number between 0 and 1. A stochastic method is used to estimate the dispersion of non-conservative radionuclides and provide concentrations in water, suspended matter and bottom sediments. These processes are formulated using kinetic transfer coe?cients, considering that exchanges of radionuclides between the liquid and solid phases are governed by a first-order reversible reaction [VI-3]. The di?erential equations which describe transfers between the three phases are expressed as follows: ??? ?? = ?????? ? ????? (VI-4) ??? ?? = ????? (VI-5) ??? ?? = ?????? (VI-6) where Cw, Cs and Cb are radionuclide concentrations in seawater, suspended matter and bottom sediments, respectively. k1m is the kinetic coe?cient describing radionuclide transfer from