Download or read book Energetics of Rare Earth Doped Uranium Oxide Solid Solutions written by Lei Zhang and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The physical and chemical properties of UO2 nuclear fuels are affected as fission products accumulate during irradiation. The lanthanides, a main group of fission products, form extensive solid solutions with uranium oxide in the fluorite structure. Thermodynamic studies of such solid solutions had been performed to obtain partial molar free energies of oxygen as a function of dopant concentration and temperature; however, direct measurement of formation enthalpies was hampered by the refractory nature of these oxides. In this work, high temperature oxide melt solution calorimetry was utilized to study the thermochemistry of various rare earth doped uranium oxide Lnx̳U1−x̳O2−0.5x̳+̳y̳ (Ln = La, Y, Nd) over a wide range of dopant concentrations and oxygen contents. The sintered solid solutions were carefully characterized to determine their phase purity, chemical composition, and uranium oxidation state, with most of the materials in the oxygen excess regime. The enthalpies of formation of Lnx̳U1−x̳O2−0.5x̳+̳y̳ were calculated from the calorimetric data. The oxidation enthalpies of these solid solutions are similar to that of UO2. The formation enthalpies from constituent oxides (LnO1.5, UO2, and UO3) become increasingly negative with addition of dopant cations and appear relatively independent of the uranium oxidation state (oxygen content) when the type and concentration of the dopants are the same. This is valid in the oxygen excess regime; thus an estimation of formation enthalpies of Lnx̳U1−x̳O2 materials can be made. The formation enthalpies from elements of hyperstoichiometric Lnx̳U1−x̳O2−0.5x̳+̳y̳ materials obtained from calorimetric measurements are in good agreement with those calculated from free energy data. A direct comparison between the formation enthalpies from calorimetric study and computational research using density functional theory was also performed. The experimental and computational energies of Lnx̳U1−x̳O2 (Ln = La, Y, Nd) generally agree within 10 kJ/mol. Since all the other doped fluorite oxides based on zirconia, hafnia, ceria, and thoria are in the oxygen deficit (oxygen vacancy formation) regime, a systematic study of these rare earth doped fluorite oxides (Lnx̳A1−x̳O2−0.5x̳) was made comparing experimental and computational results. A consistent trend suggested by both calorimetry and computation, was found for all oxygen vacancy containing systems (actinide and non-actinide oxide systems). Larger size mismatch between the smaller host cation (A4+) and the larger rare earth dopant cation (Ln3+) generally produces more stable solid solutions. The energetics of these systems is the result of competition between strain energy arising from size mismatch (endothermic) and defect association (exothermic). The formation enthalpies of Lnx̳U1−x̳O2−0.5x̳ obtained from calculation are slightly positive.