Urania (UO2) has the remarkable characteristic of exhibiting oxygen nonstoichiometry while maintaining the flourite crystal structure through oxygen vacancy formation and additions of oxygen atoms to interstitial anion sublattice sites. The convention is to denote the phase as UO_(2±x); the oxygen potential in equilibrium with UO_(2±x) determines the value of x. Characterizing the equilibrium oxygen partial pressure of urania at specific temperatures and oxygen to metal ratios (O/M) is a useful way to understand the thermochemistry and stability of the phase. The deviation from oxygen stoichiometry of urania is affected by additives or fission products that dissolve in the fluorite solid solution. Yttrium and many lanthanides (Ln), such as Ce and La, are high yield fission products or longer lived daughters of fission products and are readily dissolved in the fluorite matrix. Gadolinium is not a high yield fission or activation product; however, it can be added as a burnable poison and is soluble in urania. Gd can be present up to 10 percent by weight in fresh UO2 fuel; therefore, understanding the thermochemistry of the U_(1-y)Gd_yO_(2±x) solid solution is important for development of a comprehensive and self-consistent thermodynamic database to describe phase equilibria in oxide nuclear fuels. The objective of the effort reported here is a Gibbs energy model of the fluorite phase U_(1-y)Gd_yO_(2±x) using a three sublattice compound energy formalism (CEF) representation. The CEF for U_(1-y)Gd_yO_(2±x) can be readily integrated into more complex multicomponent models of the UO2 fluorite phase within a database framework; indeed, the model presented in this work is an extension of the one developed for UO_(2±x) by Gueneau et al.
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