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

Abstract

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 di ff usion to the environment. We present an ab initio force fi eld for the uranyl − montmorillonite interaction based on the hydrated ion model, i.e., recognizing the [UO 2 (H 2 O) 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 fi rst shell and clay oxygens. Uranyl − clay interaction sites were identi fi ed as groups of three Mg substitutions. Increasing the uranyl concentration enhances mobility due to partial surface coverage. Uranyl di ff uses by means of a hopping mechanism. The constrictivity factor, δ int , from the simulation self-di ff usion coe ffi cient of [UO 2 ] 2+ was calculated. A semiquantitative agreement with the experimental datum was obtained.