Structures of hypo-, and hyper-stoichiometric UO₂±x doped with Trivalent Rare Earth

UO₂ has been studied extensively in order to determine the effect of fission-product impurities present in solid solution in used fuel. Due to their presence, enhance the stability of the cubic fluorite structures of U₄O₉-type with respect to U₃O₈-type depending on the sintering atmosphere as compared to un-doped UO₂. Ho and Radford reported that charge compensation in the Gd doped urania can be described with formation of oxygen vacancy, oxidation of U⁴⁺ to U⁶⁺ via U⁵⁺, interstitial formation or combination of defect types. In determining fuel dissolution and radionuclide release rates, the reactivity of the UO₂ matrix and how it is modified by prior irradiation, is important. The one of the important changes expected to influence the reactivity of the fuel is the rare earth doping of UO₂ which will change the conductivity and structural properties of the UO₂ matrix. This work presents hypo-, and hyper-stoichiometric UO_2±x, and/or low and highly doped U_1-yGd_yO_2±x manufactured by mechanical blending method followed by oxidation with control the oxygen potential. Stoichiometry of all pellets was determined and distinguished into hypo-, and hyper-stoichiometric UO_2±x by analysis of using non-destructive X-ray method, conveniently. And influences of non- stoichiometric urania matrix and Gd dopant on dissolution behaviors of UO_2±x matrix were investigated by electrochemical study. The lattice contraction of pellets was observed to determine the stoichiometry conveniently, depend on Gd contents and stoichiometry. Gd doped in hyper- stoichiometric U_1-yGd_yO_2+x shows more dramatic contraction of lattice parameter depend on Gd contents, according to formation of smaller sized U⁵⁺ in matrix, even though Gd3+ are introduced. Meanwhile, larger oxygen vacancy in hypo-stoichiometry induces volumetric expansion.