Power plants use seawater for cooling. This process generates thermal brine discharges in the marine environment. The initial dilution of these discharges is studied through semi-empirical or empirical models, accepting a large number of simplifications. The phenomena involved in this fluid dynamic problem can be analyzed with Computational Fluid Dynamics (CFD) solving the three dimensional fluid motion general equations. This approach allows to study the local dilution processes and the effluent dispersion from a global perspective. In this paper, this technique has been implemented to study the discharge of a power plant in Gijón (Northern Spain). Finally, the results were compared with those given by the semi-empirical model CORMIX.