Vacancy-Mediated Increases in Brine–Salt Surface\nEnergies

Salt\nformations have been explored for the permanent isolation\nof spent nuclear fuel based on their high thermal conductivity, self-healing\nnature, and low hydraulic permeability to brine flow. Vacancy defect\nconcentrations in salt complicate fracture mechanics not driven by\ndislocation dynamics and can influence the resulting surface structure.\nClassical molecular dynamic simulations were used to simulate tensile\ntesting of salt crystals (halite) with vacancy defect concentrations\nof up to 0.5 defects/nm<sup>3</sup>. Increasing defect concentrations\nresulted in a decrease in ultimate tensile strength and fracture surface\nenergies, driven by increased surface roughness rather than changes\nin the amount of surface area. Brine–salt surface energies\nof the fractured surfaces were 0.22 to 0.26 J/m<sup>2</sup>, significantly\nhigher than values reported for atomically flat (100) surfaces at\nthe same brine composition. This change in surface energy increased\nthe brine–salt dihedral angle by ∼27°. The dihedral\nangle threshold for percolation in salt is 60°, and a 27°\nincrease due to rough fracture surfaces identifies a reduction in\nporosity percolation and a decrease in salt permeability. Therefore,\nbedded salt and salt domes may be even more stable than those previously\npredicted from dihedral angle calculations.