Radionuclide adsorption mechanism in buffer materials in high-level radioactive waste container: MD study

The buffer material plays a decisive role in preventing the radionuclide to enter into the host rock, as it is the last defense of engineered barrier system. Under very high groundwater pressure, a large amount of cations percolate through the barrier with underground water, resulting in a complicated chemical condition. Molecular dynamics simulation is performed to deeply investigate the adsorption mechanism of radionuclide species onto substituted montmorillonite (001) surface in the presence of different counterions. MD simulations exhibit three typical adsorption modes: outer-sphere complex, monodentate inner-sphere complex and bidentate inner-sphere complex. With the presence of carbonate ions and covalent cations, the U atom in uranyl can coordinate with carbonate oxygen in connection with cations to form an intensive adsorption complex with MMT surface. The thermodynamic work of adhesion between the complexes and the MMT surface is calculated to evaluate the adsorption interaction. The complexes with the carbonate and covalent cation components exhibit a relatively high adhesion with the buffer material surface.