DFT + U study of U1-yAnyO2-x (An = Np, Pu, Am and Cm) {111}, {110} and {100} surfaces

Mixed dioxides provide a means to recycle the highly radioactive transuranic elements from spent UO2 fuel. In order to gain a better understanding of such dioxides, knowledge of their geometric and electronic structures is important. However, a systematic study and comparison among uranium-actinide mixed dioxides is lacking. In this work, Hubbard U-corrected generalized gradient approximation density functional theory is used to study the surface properties of uranium-actinide mixed dioxides, where actinide = neptunium, plutonium, americium or curium. On stoichiometric surfaces, clear charge transfer is found from uranium to americium and curium, reducing them to An(III), while Np remains tetravalent. Trivalent plutonium may exist on stoichiometric uranium-plutonium surfaces, under limited conditions. On the substoichiometric (oxygen vacancy) mixed dioxide surfaces, all the transuranic elements are trivalent. Both the replacement energies of actinides into uranium dioxide surfaces, and the oxygen vacancy formation energies on uranium-actinide mixed dioxide surfaces, are strongly dependent on the An(IV)/An(III) redox potential. A linear relationship is also found between the replacement energy and the ratio of actinide to uranium in the mixed dioxides.