Effect of layer charge on the swelling of Na+, K+, Cs+ and Ca2+ -montmorillonites: a molecular dynamics simulation

Bentonite buffer is an essential component planned in the KBS-3 concept for safe disposal of spent nuclear fuel. It limits mass flow to and from the copper canister to prevent corrosion, and in the case of canister failure, it retards transport of radionuclides to bedrock, and it protects the canister in the case of large dislocations of bedrock caused by e.g. tectonic movement. Bentonite consists of montmorillonite which is the clay's main and functional component, some dissolved salts and accessory mine als. Montmorillonite is a swelling clay mineral that consists of 1 nm thick and approximately 200-400 nm wide negatively charged layers and charge compensating cations near the mineral surfaces. It has the ability to absorb water molecules into the inter layer space, resulting in swelling of the mineral, and the ability to change its cationic composition. Bentonite's beneficial properties are based on nano level reactions and structures, and therefore molecular level studies are needed regarding long term performance studies of spent fuel disposal. The aim of this work is to study the effect of layer charge on montmorillonite's swelling properties. Three different layer charges are applied to four montmorillonites (Na, Ca, K and Cs) with 16 different water contents (from 1-16 molecules per unit cell), which means about 1-3 water layers in the interlayer space. Larger basal spacings are observed with lower layer charges of Na- and Ca- montmorillonites, and swelling is greater with lower charges. Results for K- and Cs-montmorillonites will be reported later. Molecular dynamics calculations are performed on an onelayered montmorillonite particle in a periodic simulation box. The objective is to determine the interlayer thickness at equilibrium state in each case. Calculations are done using the molecular dynamics software LAMMPS with the CLAYFF interaction potential. As a result, swelling curves with respect to the number of water molecules per unit cell are determined. Comparisons to the sample system density (contribution only f om mineral layer and interlayer) and the number of interlayer water layers are made.