[en] Capability of CFDS-FLOW3D-code to model 3-D flows in Loviisa NPP primary circuit has been studied. Primary circuit has been modelled from the main circulation pumps to the inlet of the core. In the theoretical part, the flow equations and their time-averaging are introduced. Different turbulence models are presented as well as velocity profiles of turbulent flow in shear layers. The special issues of discretization of the flow equations, such as numerical diffusion, are considered. Different discretization methods, convergence criterias and gird generation are introduced. Main goal was to generate the grid for the wide and complex geometry with multi-block technique. The total amout of grid points is 131000. During analyzing Upwind and Van Leer discretization methods are used as well as ke- and RNG-ke-turbulencemodels. The computational results have been compared to the measured results of a 1/5 model scale reactor. (orig.) (21 refs.)