Molecular dynamics simulation of thermal, hydraulic, and mechanical properties of bentonite clay at 298 to 373 K

Bentonite, a fine-grained geologic material rich in smectite clay, is considered for use in the isolation of high-level radioactive waste (HLRW) because of its low hydraulic permeability, high swelling pressure, and geochemical stability. A complicating factor in this application is that heat released by nuclear waste can trigger complex coupled thermal-hydraulic-mechanical-chemical (THMC) phenomena within the barrier. Prediction of these phenomena using large-scale simulators, which typically examine problems on scales of 10⁻² to 10⁴ m, is inhibited by insufficient knowledge of the material properties of bentonite and their dependence on temperature. Here, these properties were evaluated using replica-exchange molecular dynamics (REMD) simulations of a clay assemblage containing 27 Na-smectite nanoparticles with full atomistic-level resolution solvated using 187,131 water molecules. The simulations yielded predictions of heat capacity, thermal conductivity, thermal expansivity, hydraulic conductivity, and water and ion diffusivity at temperatures of 298 to 373 K. Results showed that temperature modulates the capacity of clay barriers to transfer heat, fluids, and chemical species to different degrees. Material properties of hydrated smectite predicted on scales of tens of nanometers and nanoseconds were consistent with the properties of bentonite measured on scales of centimeters and days.