In this study, 3D numerical simulations using an Eulerian n-fluid approach of a gas-solid fluidized bed composed of very dense particles of tungsten (19,300 kg m(-3)) were carried out to examine the behavior of this suspension, especially the effects of the reduction of the fluidization column diameter on the fluidization quality. Tungsten was selected as a surrogate material of U(Mo) (Uranium molybdene) which is of interest for new nuclear fuels with limited enrichment. Comparisons between experiments and computations for the axial pressure profile of a 5 cm diameter column demonstrate the capability of the mathematical models of the NEPTUNE_CFD code to simulate the fluidization of this powder located outside the classification of Geldart. The numerical results show that the mobility into the bed of these very dense particles is very low. The reduction of the fluidization column diameter from 5 cm to 2 cm does not have significant effect on the local solid circulation but strongly decreases the axial and radial mixing of the particles due to wall-particles friction effects. These results confirm and allow to better understand the wall effects experimentally evidenced. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.