  Montmorillonite (MMT) is the main mineral component of bentonite, which is
currently proposed as a sealing material in deep geological repositories (DGRs)
for used nuclear fuel. In the Canadian program, which will utilize
copper-cladded used fuel containers, safety analysis considers the effect of
copper corrosion, during which Cu$^{2+}$ ions could potentially be adsorbed by
the surrounding MMT. In such a scenario, ion exchange between Na$^+$ and
Cu$^{2+}$ is expected. In this study, a multiscale approach that combines
electronic density functional theory (DFT) and force-field-based molecular
dynamics (MD) simulations was employed to study the effect of introducing
Cu$^{2+}$ ions to MMT. An extension to the ClayFF force field is parametrized
and validated using DFT to model how Cu$^{2+}$ interacts with clay systems. MD
simulations were performed to calculate the interaction free energies between
MMT platelets containing Cu$^{2+}$ ions (Cu-MMT) and compared them to
inter-platelet interaction energies in Na-MMT and Ca-MMT. Our calculations
suggest Cu-MMT develops swelling pressures between those of Ca-MMT and Na-MMT.
Furthermore, our MD simulations suggest that Cu$^{2+}$ has MMT interlayer
mobility that is significantly slower than that of Ca$^{2+}$.
