Experimental Thermochemistry of Select Uranyl and Neptunyl Phases

Despite their environmental importance, thermodynamic properties of uranyl and neptunyl phases remain largely unknown. In this thesis, I examine thermodynamic properties of a large family of uranium minerals from the meta-autunite group, as well as the uranyl vanadates from the francevillite group, and neptunium compounds, NpO₂ and Np₂O₅. A Calvet-type high-temperature oxide melt calorimeter was utilized to obtain enthalpies of formation of these phases.

In the first part of this dissertation I describe the process of building the calorimetry laboratory. Discovery and expansion of the large family of actinide compounds motivated the creation of the high-temperature calorimetry facility at the University of Notre Dame. In order to produce precise, reproducible, and reliable thermodynamic data, high-precision calorimetry is required. A Setaram Alexsys high-temperature oxide melt calorimeter was installed in the special temperature controlled laboratory that is necessary for proper operation of the instrument. The calorimeter was installed, stabilized, calibrated and custom tailored to the needs of work with actinide materials

The remainder of this work presents results of the experimental thermochemistry of several uranium and neptunium phases. Synthesized compounds were analyzed for phase purity using PXRD and analytical chemistry methods (ICP-OES, ICP-MS, and TGA). Their enthalpies of formation were measured using high-temperature calorimetry. The normalized charge deficiency per anion (NCDA) approach relates the thermodynamic stability of these compounds to their crystal structures, and is a useful approach for probing thermodynamic properties of materials with identical structural units. We also show experimental evidence why autunite is meta-stable relative to its lower hydrate, meta-autunite. Calorimetric data allowed us to calculate the enthalpy of dehydration of autunite, indicating that dehydration should be spontaneous. Data presented in this dissertation confirms the common occurrence of the mineral meta-autunite in Nature. Thermodynamic properties presented here are important for understanding the genesis of uranium deposits, long-term disposal of radioactive waste in a geological repository, and uranium and neptunium mobility in the subsurface.