Tight fuel lattices are characterized by quasi-periodical flow oscillations between subchannels, as first observed by Hooper (). The above mentioned phenomena are of inherently unstable nature and, even if no conclusive theoretical study on the subject have been published, the evidence shown to this point suggests that the oscillations are connected to interactions between eddy structures of turbulent flows in adjacent subchannels (). This coherent structures travel in the direction of homogeneous turbulence, in a fashion that strongly recalls the Von-Karman Vortex Street (). Analogous behaviours have been observed for arrays of arbitrarily shaped channels (), within certain range of the geometric parameters. Numerical simulation of these phenomena is challenging, and a completely satisfactory explanation of their cause is not available. A modelling for these phenomena is at least problematic to achieve since they are turbulence driven. This paper aims to address the use of POD (Proper Orthonormal Decomposition) to reduce the Navier-Stokes equation to a set of ordinary differential equations ().The set of ODE is solved numerically, and a series of comparison with experimental and numerical data is offered, in order to test the predicting capability of the models proposed.