Study of (Zr,U)SiO₄ Solid Solutions as Phases of Interest for Fuel Interaction in Corium

In the first stage of a severe nuclear accident, the loss of cooling leads to a complex solid-liquid mixture known as in-vessel corium, and consequently to high-temperature reactions between the fuel, UO₂ and surrounding materials. In a second step of the accident, if the vessel is breached, this corium can flow out and react with concrete or cement. The silicate Zr_1-xU_xSiO₄ known as Chernobylite (present in the Chernobyl corium), can then be formed by reaction between the mixed oxide Zr_1-xU_xO₂ and the silica SiO₂ [1]. In order to predict the heat exchanges as well as the phases formed in the corium, severe accident codes are coupled with thermodynamic calculation software. In these databases, the UO₂-ZrO₂-SiO₂ pseudo-ternary diagram is one of the most important to be accurately described, to predict the phases formed in the out of vessel corium. However, the solubility limit of uranium in zircon (ZrSiO₄) and the thermodynamic properties of chernobylite Zr_1-xU_xSiO₄ are still poorly known. Therefore, the preparation of pure samples and their complete and advanced characterization are required to solve this problem.
On the basis of the optimized synthesis of zircon ZrSiO₄, the preparation of Zr_1-xU_xSiO₄ solid solutions by wet chemical route has recently been optimized, thanks to hydrothermal synthesis methods (250°C, 7 days) [2,3]. It resulted to a uranium incorporation rate in zircon of up to about 62% (a value much higher than that reported in the literature so far for synthetic samples generally prepared by powder mixing, i.e. 20-25%). However, HR-PXRD diagrams showed that the samples prepared contained amorphous silica (continuous background) and uranium oxide UO_2+x as secondary phases. The analysis of unit cell parameters allowed the understanding of the partitioning of U and Zr in these multiphase systems.
The detailed multiparametric study developed allowed the optimization of the final samples. In particular, it has been shown that the pH of the reaction medium (optimized to 1.6 to 1.8) as well as the heating time (7 to 14 days) are likely to improve the purity of the raw samples obtained from the hydrothermal treatment by reducing the uranium oxide UO_2+x content.
Since the prepared samples contained traces of residual oxides, a purification step by selective dissolution (succession of washing steps in acidic and basic media) was also optimized. It was adapted to the chemical composition of the oxide in question (ZrO₂ is much more refractory than UO₂, even in acidic media). First results showed the quantitative elimination of both oxide-based secondary phases. Finally, pure (Zr,U)SiO₄ solid solutions were prepared by this method.
These samples can be now characterized from a structural and thermodynamic point of view (calorimetry, solubility). Calorimetric measurements and solubility experiments will be developed to determine the variations of the enthalpy of formation and free Gibbs energy, respectively. Variations in the entropy of formation will be obtained by direct combination of these two values. In addition, detailed characterization of the samples will be performed using Raman and HERFD spectroscopy to assess the speciation of uranium in the zircon structure within these samples. All of these results will serve as input data for the initial modeling of the UO₂-ZrO₂-SiO₂ ternary diagram, which is essential for predicting the behavior of corium when reacting with cement or concrete.

[1] Anderson, E. B. et al. Radiochimica Acta, 1993, 60, 149-151.
[2] Barral, T. et al. Dalton Trans., 2023, 52, 10023-10037.
[3] Estevenon, P. et al. Dalton Trans., 2024, 53, 13782-13794.