Investigation of the Cs2(Mo,Te)O4 Solid Solution and Implications on the Joint Oxyde-Gaine System in Fast Neutron Reactors

The formation of a thin layer, the so-called Joint Oxyde-Gaine (JOG), between the (U,Pu)O2 fuel pellets and the cladding has been observed in fast neutron reactors, due to the accumulation of volatile fission products. Cs2MoO4 is known to be one of the major components of the JOG, but other elements are also present, in particular tellurium and palladium. In this work, an investigation of the structural and thermodynamic properties of Cs2TeO4 and Cs2Mo1–xTexO4 solid solution is reported. The existence of a complete solubility between Cs2MoO4 and Cs2TeO4 is demonstrated, combining X-ray diffraction (XRD), neutron diffraction (ND), and X-ray absorption spectroscopy (XAS) results. High-temperature XRD measurements were moreover performed on Cs2TeO4, which revealed the existence of a α–β phase transition around 712 K. Thermal expansion coefficients were also obtained from these data. Finally, phase equilibra points in the Cs2MoO4–Cs2TeO4 pseudobinary phase diagram were collected using differential scanning calorimetry and used to develop a thermodynamic model for this system using a regular solution formalism.
The existence of a complete solid solution between Cs2MoO4 and Cs2TeO4 is demonstrated combining XRD, ND, and EXAFS data. The hexavalent states of Mo and Te in the Cs2Mo1−xTexO4 solid solution have been confirmed by XANES. The existence of a phase transition at (712 ± 5 K) in Cs2TeO4 (from an orthorhomic to an hexagonal structure) is revealed. Phase equilibria measurements in the Cs2MoO4−Cs2TeO4 phase diagram are reported, and a regular solution model has been developed.