Hydration and Diffusion Mechanism of Uranyl in Montmorillonite Clay: Molecular Dynamics Using an Ab Initio Potential

Permanent geological repositories lined with bentonite, a montmorillonite-containing clay, is one of the options considered for the storage of high-level radioactive waste. If the fuel rods were dissolved by a water leak, the clay would exchange its cations with the radioactive cations, slowing down their diffusion to the environment. We present an ab initio force field for the uranyl-montmorillonite interaction based on the hydrated ion model, i.e., recognizing the [UO2(H2O)(5)](2+) as the cationic species. This new interaction potential was used to run molecular dynamics simulations of the hydrated clay system. The uranyl aqua ion formed outer-sphere complexes with the clay layers, in agreement with EXAFS data. The hydrate is strongly bound, forming 1.4 hydrogen bonds between the first shell and clay oxygens. Uranyl clay interaction sites were identified as groups of three Mg substitutions. Increasing the uranyl concentration enhances mobility due to partial surface coverage. Uranyl diffuses by means of a hopping mechanism. The constrictivity factor, delta(int), from the simulation self-diffusion coefficient of [UO2](2+) was calculated. A semiquantitative agreement with the experimental datum was obtained.