Abstract
Available data on the dependence of the equilibrium chemical potential of oxygen on degrees of doping, z, and non-stoichiometry, x, y, in U1-z Ln z O2+0.5(x-y) fluorite solid solutions and data on the dependence of the lattice parameter, a, on the same variables are combined within a unified structural-thermodynamic model. The thermodynamic model fits experimental isotherms of the oxygen potential under the assumptions of a non-ideal mixing of the endmembers, UO2, UO2.5, UO1.5, LnO1.5, and Ln 0.5U0.5O2, and of a significant reduction in the configurational entropy arising from short-range ordering (SRO) within cation-anion distributions. The structural model further investigates the SRO in terms of constraints on admissible values of cation coordination numbers and, building on these constraints, fits the lattice parameter as a function of z, y, and x. Linking together the thermodynamic and structural models allows predicting the lattice parameter as a function of z, T and the oxygen partial pressure. The model elucidates contrasting structural and thermodynamic changes due to the doping with LaO1.5, on the one hand, and with NdO1.5 and GdO1.5, on the other hand. An increased oxidation resistance in the case of Gd and Nd is attributed to strain effects caused by the lattice contraction due to the doping and to an increased thermodynamic cost of a further contraction required by the oxidation.
Highlights
One important aspect of a safety case for a geological repository for the high-level nuclear waste (e.g., UO2-based spent nuclear fuel, SNF) is to characterize the corrosion behaviour of and the consequent radionuclide release from the disposed wastes, after waste containers will be eventually breached due to corrosion
The data on the dependence of the oxygen potential of doped UO2 solid solutions on the composition, temperature and nonstoichiometry are linked to the data on the dependence of the lattice parameter on composition and non-stoichiometry within the same model frame
As the lattice parameter can be predicted as a function of synthesis conditions, its measuring provides a test for an attainment of the thermodynamic equilibrium in a particular experiment
Summary
One important aspect of a safety case for a geological repository for the high-level nuclear waste (e.g., UO2-based spent nuclear fuel, SNF) is to characterize the corrosion behaviour of and the consequent radionuclide release from the disposed wastes, after waste containers will be eventually breached due to corrosion. The unique chemical conditions are characterized by 1) generally reducing conditions due to a significant amount of hydrogen produced due to the anoxic corrosion of metallic waste containers, 2) the presence of locally formed oxidizing conditions caused by radiolytic products such as H2O2, and 3) complex groundwater chemistry. The radiolytic oxidants evoke locally oxidizing effects at the spent fuel surface leading to an oxidative dissolution of the waste, due to the oxidation of poorly soluble U(IV) to soluble U(VI) (Shoesmith, 2000; Eriksen et al, 2012; Shoesmith et al, 2013; Bosbach et al, 2020; Hansson et al, 2021)
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