Preparation and Characterization of (U,Zr)O₂ Solid Solutions, as Corium Phases of Interest

In the event of a severe nuclear accident, the loss of cooling leads to high-temperature reactions between the fuel, UO₂, and surrounding materials. The fuel can then react with the zirconium alloy cladding and the steel vessel to form a complex solid+liquid mixture known as in-vessel corium. Understanding the formation and complete characterization of the U_1-xZr_xO₂ phases is essential for predicting the heat exchanges as well as the phases formed in the corium. However, the U-Zr-O system is not fully understood from a thermodynamic point of view. Therefore, in order to improve our knowledge of the formation of such corium, the preparation of pure U_1-xZr_xO₂ samples and their complete and advanced characterization are required.
For this purpose, the oxide phases were prepared by hydroxide precipitation, which guarantees better homogeneity than dry synthesis processes. According to the literature, this method produces nanometric, highly reactive powders, favoring the formation of cubic solid solutions with high zirconium content at significantly lower temperatures than conventional dry chemical routes [1-3].
The hydroxide route offers several key advantages for the preparation of (U,Zr)O₂ solid solutions. First, it ensures a quantitative and homogeneous precipitation of cations, which allows an improved elemental distribution in the powders. Obtaining nano-sized powders could also favor the formation of metastable phases at lower temperatures than conventional methods and improve their efficient conversion during calcination. Thus, after precipitation, washing and drying, the initial hydroxide precursors were thermally converted at temperatures ranging from 800°C to 1700°C
The formation of a fluorite-like phase at 800°C was confirmed by HR-PXRD, which is not consistent with the phase diagram. The recorded diagrams confirmed the formation of this cubic (U,Zr)O₂ solid solution, while excluding the presence of a secondary phase for contents of zirconium of 60 mol.% or less. The unit cell parameters refined by Le Bail's method showed a linear decrease, consistent with the replacement of U by Zr within the fluorite structure in such slightly overstoichiometric samples. Such a result is unexpected and probably reflects the metastable nature of the prepared solid solutions. In fact, it could be related to the nanometric nature of the prepared powders.
Phase demixing was observed upon heating at 1250°C, leading to the formation of a cubic UO₂ + tetragonal ZrO₂ mixture. This results in a significant decrease in the incorporation limit of Zr in UO₂ at this temperature (≈ 10 mol.%, a value very slightly higher than that expected from the literature). Annealing at 1700°C significantly improves the incorporation rate of Zr into the ZrO₂ structure (≈ 28 mol.%).
These results are particularly important for the study of the thermodynamic stability of corium. The preparation of this series of pure compounds paves the way for an improved revision of the U-Zr-O phase diagram as well as for the determination of the associated thermodynamic properties. In this context, calorimetric measurements will be performed to determine the variations in enthalpy of formation. At the same time, solubility measurements will be used to determine the associated Gibbs energy and then, by combination, variations in the entropy of formation. In addition, detailed characterization of the samples will be performed using Raman and HERFD spectroscopy to assess the speciation of uranium within these solutions. All these results will be used to clarify various aspects of the U-Zr-O ternary diagram.

[1] Martinez, J. et al. J. Nucl. Mater., 2015, 462, 173-181.
[2] Clavier, N. et al., Chem. Phys. Chem., 2017, 18, 2666-2674.
[3] Massonnet et al., Inorg. Chem., 2023, 62, 7173-7185.