Thermodynamics of Material Containing Actinides: Garnets, Metal Uranates, and Uranyl Peroxide

Abstract

As a part of actinide science, the thermodynamics of actinide compounds is important in understanding structures and properties of actinide compounds, evaluating thermal and environmental stability, predicting the alteration of actinide-containing nuclear waste, and guiding synthesis and processing of actinide materials.

Garnet may provide a promising ceramic nuclear waste host that stabilizes radionuclides into a more durable form for long-term storage and/or geologic disposal. Thus, I designed and studied garnet matrices in terms of structural and thermodynamic properties for nuclear waste immobilization. In Chapters 2, 3, 4, and 5, simple yttrium iron garnet and complex ferric garnet matrices were substituted with Ce, Th, and/or U. The structural differences introduced by the lanthanides/actinides were studied in depth through X-ray and Mössbauer spectroscopy. To understand the substitution of actinides in garnet and its thermodynamic stability, the enthalpies of formation were derived and the enthalpies of substitution were calculated. The impacts of actinide substitution on the thermodynamics of garnet systems were discussed in detail.

Metal uranates, MgUO₄, CrUO₄, and FeUO₄, were synthesized and characterized using X-ray spectroscopy and calorimetry. U⁵⁺ was found in CrUO₄ and FeUO₄, and U⁶⁺ in MgUO₄. The enthalpy of formation, Δ H_f,ox (kJ/mol), for the three compounds at 25 ºC were derived: MgUO₄ (-33.8 ± 1.7), CrUO₄ (-31.4 ± 0.9), and FeUO₄ (-9.8 ± 1.1). ΔH _f,ox (kJ/mol) of CrUO₄ was reported for the first time and that of FeUO₄ was contributed to its scant thermodynamic reference data. More importantly, the role of U⁵⁺ in stabilizing these U⁵⁺ uranates is discussed in Appendix B.

Uranium peroxides, metastudtite and studtite, can be formed when UO ₂ based nuclear fuels are exposed to water during geological disposal or as a result of reactor accidents. Careful calorimetric work was done on a rare uranyl peroxide mineral, metastudtite, to study its thermal decomposition and enthalpy of formation (Chapter 6). Stepwise decomposition in O₂ up to 1000 ºC was observed and enthalpies of reactions associated with each step were measured. Derived ΔH_f,ox (kJ/mol) for metastudtite in different conditions are 15.8 ± 1.7 with respect to γ-UO₃, H₂O, and O₂, -82.3 ± 1.7 with respect to γ-UO₃, H₂O, and H ₂O₂, and -16.5 ± 2.0 from U₃O₈, H₂O, and O₂. The thermodynamic data confirm the irreversible transformation from studtite to metastudtite, and show that metastudtite can be a major oxidized corrosion product at the surface of UO₂ and contribute a significant pathway to dissolution.