Application of the CALPHAD method to the thermodynamic modeling of a miscibility gap in the U-Nd-O phase diagram
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Application of the CALPHAD method to the thermodynamic modeling of a miscibility gap in the U-Nd-O phase diagram Giannina Dottavio1, Yves Pontillon1, Lionel Desgranges2, Christine Gueneau3, J-C. Dumas2 1. CEA, DEN, DEC, SA3C, LAMIR, F-13108 Saint Paul lez Durance, France. 2. CEA, DEN, DEC, SESC, LLCC, F-13108 Saint Paul lez Durance, France. 3. CEA, DEN, DPC, SCCME, LM2T, F-91191 Gif-sur-Yvette, France. ABSTRACT During neutron irradiation in nuclear power plants, uranium dioxide (UO2), 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 UO2 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 (U0.72Nd0.28)O2, 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. INTRODUCTION A great number of fission products are created in nuclear oxide fuels due to neutron irradiation, with many of them being soluble in the crystallographic structure of the ceramic [1]. It is important to investigate the effects of incorporating these FPs into the fluorite fuel lattice since the presence of additional species in the fuel matrix can affect the overall characteristics of the fuel and thus its performance. Among these soluble species, neodymium deserves special attention. Firstly, because it is the most abundant soluble fission product, but especially because of the miscibility gap recently detected in the U-Nd-O ternary system[2], which is contrary to the more traditional assumptions affirming that Nd has a high solubility level in UO2 [3]. It seems that the appearance of a miscibility gap in doped-UO2 is a complex phenomenon. Currently, the conditions leading to its appearance in lanthanide-doped oxide fuels have yet to be identified. Therefore, the aim of this work is to characterise this miscibility gap in order to obtain an accurate explanation about its appearance and stability range. This was done by a theoretical approach which consists in thermodynamically assessing the U-Nd-O ternary system by using a model developed with the compound ene
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