3D modeling of the thermal hydraulical processes in a fuel assembly head means a great challenge for the CFD technique due to the complexity of its structure and the flow domain. On the other hand, this field is of great importance since detailed knowledge on mixing processes in the assembly heads and calculations on the signals of the thermocouples positioned just above the heads would give very significant information for the safety analyses connected to the power upgrading of nuclear power plants. Therefore development of a complex fuel assembly model was started in the Institute of Nuclear Techniques of the Budapest University of Technology and Economics in the near past. In this paper, the fuel assembly head model, the sensitivity study of it, calculations and results are presented. The main goal of our work is investigating the signal of the thermocouples which are placed just above the fuel assemblies. The calculations were performed with consideration of four kinds of different fuel assemblies. The inlet velocity and temperature fields were calculated by the COBRA subchannel code of the Paks Nuclear Power Plant of Hungary. With all kind of fuel assemblies, calculations were performed with assumptions of normal symmetrical and highly asymmetrical heat source profiles of inner assemblies and assemblies positioned beside absorber elements.

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