Axial coolant flow inside a tightly packed rod bundle presents a complex behavior. Experimental analysis have clearly shown that when reducing the pitch-to-diameter ratio (P/D) the turbulence field in rod bundles deviates significantly from that in a circular tube. Moreover for tight configurations the existence of large-scale periodic flow oscillations has been shown, which is responsible for the high inter-sub-channel heat and momentum exchange (). A complete understanding of these oscillations has still to be achieved; the evidence shown to this point suggests that the oscillations are connected to interactions between eddy structures of turbulent flows in adjacent sub-channels (). The aim of this paper is to show that the adoption of an Unsteady Reynolds Averaged Navier Stokes (URANS) methodology allows to reproduce these oscillations in the numerical model and to obtain an accurate prediction of the averaged statistics. The validity of the methodology is assessed by comparing computational results with the experimental data of Krauss (). An improved anisotropic k-ε model and the traditional k-ε model, implemented in the code Star-CD v3.26, have been used as turbulence models. Results in good agreement with experimental results have been obtained with the anisotropic k-ε model.