Application of the CALPHAD method to the thermodynamic modeling of a miscibility gap in the U-Nd-O phase diagram

摘要

During neutron irradiation in nuclear power plants, uranium dioxide (UO_2), the most used nuclear fuel, changes gradually its chemical composition because of the incorporation of new chemical elements which are created by fissions and named Fission Products (FP). As a consequence, the fluorine-type crystalline structure and its lattice parameters may also be modified. In order to better understand this behavior, neodymium-doped UO_2 ceramics have been prepared with the aim to simulate the crystallographic matrix of irradiated fuels, since Nd is one of the most abundant FP. In a previous work, high temperature X-ray diffraction was performed on a sample (U_(0.72)Nd_(0.28))O_2, annealed under reducing conditions. The diffractograms evidenced, for the first time, the existence of a miscibility gap in the U-Nd-O system. In this paper, we present the first results of a thermodynamic modeling of the ternary system U-Nd-O based on the CAlculation of PHAse Diagrams (CALPHAD) method, in order to obtain a complete description of this miscibility gap. The very first results of this modeling seem to confirm the presence of a region presenting two FCC (fluorite) phases (instead of a single solid solution, which is expected from literature). At room temperature, the gap appears from a Nd content as small as about 0.02 at. % and an O/M ratio slightly lower than 2.