S. Perez-Conesa, J. M. Martínez and E. Sánchez Marcos
Department of Physical Chemistry
University of Seville
41012 Seville, Spain
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Knowledge of actinide aqueous solution confined in smectite clay interlayers are crucial when designing permanent geological high level radioactive waste facilities. The actinide ions are sealed using montmorillonite containing clays to prevent radionuclide scape1. A computational study of [UO2]2+ (aq) hydration structure and diffusion inside montmorillonite clay was accomplished. The first actinide-clay ab initio interaction potential has been created based on the Hydrated Ion concept2,3. Additionally it is the first application of the hydrated ion model to a confined medium chemical problem. The developed potential was used to carry out NPT classical molecular dynamics simulations for Montmorillonite clay containg [UO2]2+ with two possible interlayer water contents. The z-density profiles reflect the formation of stable pentaaquo uranyl outer-shell complexes in agreement with EXAFS studies3 without any external imposed constraint. The complex interacts with the clay by the formation of 1.4 hydrogen bonds between its first hydration shell and the surface. Groups of three Mg substitutions were identified as strong interaction sites of the pentahydrate. The constrictivity factor ,δint, calculated from the self-diffusion coefficients agrees semiquantitatively with experiment. Diffusion is enhanced by an increase of concentration of uranyl in the clay interlayer.
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