Thermodynamic model of the oxidation of Ln-doped UO2

Ln-doped UO2 is often considered as a model system of spent nuclear fuel (SNF) helping to reveal effects of fission and activation products on its chemical stability. Comparing thermodynamics of UO2-UO3 and LnO(1.5)-UO2-UO(3 )systems provides a means to understand the phenomenon of an increased resistivity of Ln-doped UO2 to oxidation in air relative to pure UO2. Here a thermodynamic model is developed and is applied to investigate detailed phase changes occurring along the oxidation of Ln-doped fluorite to U3O8. The study proposes that an enhanced resistivity to oxidation of Ln-doped UO2 is likely caused by a thermodynamically driven partitioning of Ln between a fluorite-type phase and a U3O8 polymorph, which at ambient temperatures becomes hindered by slow diffusion.

Automated summary

Ln-doped UO2 is commonly used as a model system for studying the effects of fission and activation products on the chemical stability of spent nuclear fuel. This study compares the thermodynamics of UO2-UO3 and LnO(1.5)-UO2-UO3 systems to understand the increased resistance of Ln-doped UO2 to oxidation compared to pure UO2 in air. The findings suggest that the enhanced resistivity to oxidation in Ln-doped UO2 is likely due to a thermodynamically driven partitioning of Ln between a fluorite-type phase and a U3O8 polymorph, hindered by slow diffusion at ambient temperatures.