Molecular dynamics simulation of the diffusion of uranium species in clay pores

Highlights: ► We investigated the diffusion behavior of Uranium species in montmorillonite pores using MD technique. ► Our results indicated that different uranium species have distinct diffusion coefficients in clay pores. ► The negative charged uranium complexes must be addressed in the safety assessment for geological disposal of nuclear wastes. -- Abstract: Molecular dynamics simulations were carried out to investigate the diffusive behavior of aqueous uranium species in montmorillonite pores. Three uranium species (UO22+, UO2CO3, UO2(CO3)22−) were confirmed in both the adsorbed and diffuse layers. UO2(CO3)34− was neglected in the subsequent analysis due to its scare occurrence. The species-based diffusion coefficients in montmorillonite pores were then calculated, and compared with the water mobility and their diffusivity in aqueous solution/feldspar nanosized fractures. Three factors were considered that affected the diffusive behavior of the uranium species: the mobility of water, the self-diffusion coefficient of the aqueous species, and the electrostatic forces between the negatively charged surface and charged molecules. The mobility of U species in the adsorbed layer decreased in the following sequence: UO22+ > UO2CO3 > UO2(CO3)22−. In the diffuse layer, we obtained the highest diffusion coefficient for UO2(CO3)22− with the value of 5.48 × 10−10 m2 s−1, which was faster than UO22+. For these two charged species, the influence of electrostatic forces on the diffusion of solutes in the diffuse layer is overwhelming, whereas the influence of self-diffusion and water mobility is minor. Our study demonstrated that the negatively charged uranyl carbonate complex must be addressed in the safety assessment of potential radioactive waste disposal systems