Non-ideal thermodynamics of solid solutions can greatly impact material degradation under extreme conditions. Conventionally, the thermodynamic non-ideality is thought to have structural causes (e.g., local ordering, clustering). However, in this talk, we will show a recent investigation on an actinide silicate solid solution system (coffinite-thorite, USiO<sub>4</sub>-ThSiO<sub>4</sub>), which demonstrates that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, usually referred to as the random distribution. Such thermodynamic non-ideality, described by a subregular solid solution model, has a profound impact on the stability field of this solid solution in nature or under relevant nuclear waste repository conditions. Specifically, the silicate system we studied here belongs to the zircon structure that has potentials to accommodate actinides and has been proposed to immobilize plutonium. To study the thermodynamics of the USiO4-ThSiO4 solid solution, which is important for evaluating the long-term stability of immobilizing Pu or minor actinides in the zircon structure, we performed high temperature calorimetry, neutron total scattering, density functional theory calculations, and multi-component thermodynamic modeling. We were able to i) explain that the non-ideality is primarily due to an electronic effect from the removal of An 5f-O 2p covalent interaction, and ii) evaluate the impact of such non-ideality on the degradation of such solid solution phases with respect to elevated temperatures and a wide range of redox conditions. The outcome of our work may shed light on the geochemical behavior of other rare-earth elements and actinides bearing solid solution minerals, whose thermodynamic stability may also be controlled by the electron-driven enthalpic and configuration-driven entropic effects.

Impact of non-ideal mixing of U and Th on the mineral degradation of coffinite-thorite solid solution

ACS Spring 2022

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