CFD analysis of temperature deviations in Gd assembly heads

A CFD model for the head part of the VVER-440 fuel assemblies with Gd burnable poison has been developed. Using this model calculations are performed for some recent Gd assemblies of the Paks NPP with the code ANSYS CFX. The temperature distributions in the assembly heads are determined and the in-core thermocouple signals are evaluated. The computed signals are compared to the in-core measured data of Paks NPP. The study shows that the deviations between the in-core thermocouple signals and the cross-sectional average temperatures at the measurement level depend on the burn-ups of the fuel assemblies. These deviations are significant in the cases of low burn-ups and negligible in the cases of higher burn-ups. This behavior is caused by the depletion of Gd from the three fuel rods near the assembly center.