Due to the high activities and hazardous risks associated with heavy actinides such as plutonium (Pu), thermodynamic properties of many of these materials are poorly constrained. In a wide collaborative effort, we performed direct high-temperature calorimetric measurements on well-characterized plutonium oxide (PuO2±x) solid phases, from which we obtained key thermochemical parameters, such as drop solution enthalpies, enthalpies of oxidation, and thermal stability at elevated temperature. Particularly, the first reported drop solution enthalpy (in molten sodium molybdate at 700 °C) of stoichiometric PuO2, -46.04 ± 3.75 kJ/mol, provides an important benchmark for future high-T calorimetry experiments on Pu complex oxide systems. In addition, such values are compared to drop solution enthalpy values of other lighter actinides (Th, U, and Np) oxides. These thermochemical properties fill the knowledge gaps of these basic actinide oxide compounds and can be used to benchmark theoretically derived parameters. Lastly, due to the susceptibility of PuO2 to multiple metal valence states and non-stoichiometry by surface modifications, size effects on the charge of Pu and stoichiometric of oxides were simulated by studying nanosized CeO2-x, because of the similarities of Ce and Pu in sizes and charges. The lattice parameters, local structures (oxidation states and bondings), surface/bulk energies were characterized by Synchrotron X-ray diffraction, X-ray absorption spectroscopy, differential scanning calorimetry, and high-T oxide melt calorimetry, respectively, for CeO2-x with sizes from ~ 2 nm to ~ 100 nm. The experience gained on non-rad systems and above discoveries will be aligned with future work on PuO2±x systems.

High-temperature drop-solution calorimetry of PuO2: Preliminary studies

2019 Spring National Meeting

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