[en] Power upgrading of WWER-440 type reactors are being planned in several countries. From the point of view of both the planning and the licensing the detailed knowledge of the thermal-hydraulics processes in the fuel assemblies is essential. In the last years the evolution of the commercial computational fluid dynamics (CFD) codes and the growth of the computer capacities have enabled the application of the CFD codes in this field. Towards to get to know the thermal hydraulics processes in a WWER-440 fuel assembly the development of a complex fuel bundle model is under way in the Institute of Nuclear Techniques of the Budapest University of Technology and Economics. In the paper our WWER-440 fuel bundle models and results of our calculations were presented. At first a 240 mm long and - considering the symmetries - 60 degree segment model of fuel bundle was developed using the ANSYS ICEM software. The model geometry contains the 240 mm length part of 24 fuel pins, the central tube, the inner wall of the shroud and one spacer grid. Four different fine meshes were developed and wide mesh sensitivity studies were performed. Using the 240 mm long model a full length (2420 mm) fuel bundle model was built, which involves the 24 fuel pins, the central tube, the inner wall of the shroud and ten spacer grids. The model contains near to 7 millions volume elements. With this model calculations were carried out using ANSYS CFX 10.0 CFD code. In the calculations a non-profiled fuel assembly of 3.6% enrichment was investigated which was in operation in the PAKS NPP. The computations were accomplished using several RANS turbulence models and the effects of choosing different turbulence models were in details investigated. For the same assembly calculation was performed with the COBRA subchannel code too. The results of the CFX and the COBRA calculations were also compared (Authors)