[en] The present paper documents the CFD code validation activity carried out at the University of Pisa. In particular, the ANSYS CFX-10.0 code was used to simulate one of the experiments conducted at the ROCOM mixing test facility (FZD, Germany), that reproduced the injection of a de-borated slug in one cold leg of a pressurized water reactor (simulated by a salt tracer) with all circulation pumps at steady-state operation. The calculations were run on several grids obtained through different meshing strategies and having different sizes. The numerical results, in terms of normalized concentration of the transported passive scalar in the downcomer and at the core inlet, were compared against corresponding values obtained through experimental measurements of electrical conductivity in the ROCOM facility. Such comparison resulted in a general good qualitative agreement between simulations and experiments, while some discrepancies were evidenced from a quantitative point of view. (author)