HYDROXYLATION-INDUCED SURFACE STABILITY OF ANO2 (AN = U, NP, PU) FROM FIRST-PRINCIPLES

The storage, in pools, and geologic disposal on spent nuclear fuel (UO₂) requires an understanding of the mechanisms of actinide dioxide surfaces with water. As a first step the relative stabilities of clean and hydroxylated surfaces of three actinide dioxides (AnO₂, An = U, Np, Pu) have been investigated using first-principles methods within the DFT+U framework. In the case of the clean surfaces, the calculated surface energies are consistently the lowest for the (111) surface for all three actinide dioxides, followed by the (110) and (100) surface energies. In the case of UO₂, for instance, the calculated surface energies are 0.78, 1.05, and 1.47 J/m² for the (111), (110), and (100) surfaces, respectively. Therefore, the well-established surface energy trend for metal-dioxides with fluorite structure is reconfirmed by the DFT+U approach: (111) < (110) < (100).

The adsorption of dissociated water has a dramatic effect on the relative stability of the surfaces. Dissociated water, at one monolayer coverage, is adsorbed preferentially onto the (100) surface for all three AnO₂ systems. In the case of UO_2, for instance, the water adsorption energy on the (100) surface (-1.34 J/m²) is almost four times higher than the adsorption energy on the (111) surface (-0.35 J/m²), and almost twice as large as the adsorption energy on the (110) surface (-0.77 J/m²). Similar trend in the adsorption energies is observed for both NpO₂ and PuO₂. The calculated hydroxylated surface energies indicate that with a single monolayer coverage of H₂O the (100) surface is more stable than either the (111) or the (110) surfaces. Consequently, the morphology of hydroxylated AnO₂ crystallites will be no longer dominated by the (111), but rather the (100) facets.