Drivers of stability in solid solution nuclear ceramics and critical metal minerals

Natalie S Yaw

Realistic nuclear materials incorporate a variety of elements within a single crystal structure, forming solid solutions with material properties that can vary significantly from that of their endmembers. The same is true for critical metal minerals, which can incorporate a diverse array of lanthanides and actinides into one phase. Understanding how these mixing elements combine to (de)stabilize these phases enables a deep understanding of structure-property relationships which influence long-term behavior. The aim of this dissertation is to describe these structure-property relationships as they pertain to the stability of solid solution nuclear ceramics and critical metal minerals and use them to construct predictive models which target optimized compositional space or processing conditions. Herein, materials simulating nuclear fuel, waste, and minerals are synthesized and their structural and thermodynamic properties characterized. In chapter two, the formulation of general ceramic materials is studied, linking precursor choices to changes in morphology and porosity. Uranium mononitride (UN) is the subject of chapter three, where anomalous thermal expansion was detected under oxidizing conditions. Although the formation of a U(N,O) solid solution is occurring under these conditions, it was not the cause of the increased expansion. Cation mixing is the explicit focus in the studies on brannerite- and fluocerite-type materials. Highly complex solid solutions are synthesized and their structures characterized. Thermodynamic parameters were measured using high-temperature oxide-melt solution calorimetry and estimated using volumetric mismatch when direct measurements were not possible. Interaction parameters between cations were extracted and used to understand stability fields and model geochemical behavior. In brannerite-type materials, highly favorable cation-cation interactions were found between Ce-, U-, and Th- types, which stabilized the materials and drove homogeneous sample formation. In fluocerite-type materials, unfavorable interaction parameters, dictated primarily by increasing lattice strain from smaller lanthanides, were found to drive fractionation in hydrothermal conditions. These projects aim to both improve processing conditions to target ceramic materials and understand the forces which drive homogeneity and separation. Advanced X-ray and calorimetric techniques are integrated with computation and modeling in most chapters, to expand the predictive power of these studies. Results from this dissertation include predicting stable compositional space for complex wasteforms, predicting geological conditions which lead to highly fractionated minerals, and explaining the anomalous expansion of advanced ceramic fuels.

Drivers of stability in solid solution nuclear ceramics and critical metal minerals

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