The flow distribution in a condensate demineralizer vessel of a nuclear power plant is studied using the computational fluid dynamics (CFD) approach. The model simulates the flow through the packed resin bed installed in the vessel as well as the exit of flow through the porous resin retention assembly at the bottom of the vessel. The computational model is subsequently revised to assess the impact of a proposed modification to the retention assembly to enhance drainage of the vessel and minimize unwanted resin separation during resin bed regeneration. The subject model has been developed using the ANSYS ICEM CFD meshing tool and the FLUENT 6.3 CFD software as well as associated postprocessing tools. Comparisons of flow patterns in the vessel resin beds prior to and with the modification demonstrate a sharp increase in the flow rate at the end walls of the vessel, thus resulting in accelerated depletion of resin in high-velocity areas and nonuniform consumption of resin inventory. The computational results are also compared with a theoretical analysis of the basic process.