99 Using Eq. IV-22, Eq. IV-21 is simplified as: ?????,? ? ?? = ?????(??,?? ??? ? ??,?? ) + ????,? ? ??(???) ? ????,? ? ????(???) ? ?????,?? (IV-29) where: ??? = ?????? ???????????(???)? ?????? ? ??? ????(???)? ??? ? ??? (IV-30) IV-4.2. Lagrangian model In the Lagrangian model a release of radioactivity is simulated by a large number of particles, with each of them transporting an equal amount of activity. The same equations were used as for the Eulerian model but for only one characteristic fraction of sediments. The particles are transported by currents, turbulent di?usion and they can settle with sediment particles. The turbulent di?usion, transfer of activity between solute, particulate and bottom phases and decay are described by stochastic methods [IV-10]. In order to simulate radioactivity transport a Random Dispersion Model (RDM) was used where positions of particles are simulated as a random Markov process. The equations describing increment of particle position over each time increment dt are given by: ?? = ??? + ?? ?? ?? + ?2??? (IV-31) ?? = ??? + ?? ?? ?? + ?2??? (IV-32) ?? = ??? + ???? + ??? ?? ?? + ?2???? (IV-33) where u, v and w are velocity components on coordinate axis (x, y, z), and (Rx, Ry, Rz) are random variables with zero mean and variance dt.