HYDROXYLATION-INDUCED SURFACE STABILITY OF ANO2 (AN = U, NP, PU) FROM FIRST-PRINCIPLES
RAK, Zsolt, Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, EWING, Rodney C., Department of Geological Sciences, Stanford University, Stanford, CA 94305-2115 and BECKER, Udo, Earth and Environmental Sciences, University of Michigan, 2534 C.C. Little Building, 1100 North University Ave, Ann Arbor, MI 48109-1005, rakzsolt@umich.edu
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 UO2, 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.
