Effect of compaction on bisulfide diffusive transport through MX-80 bentonite

Abstract

Canada's deep geological repository (DGR) design includes an engineered barrier system where highly compacted bentonite (HCB) surrounds the copper-coated used fuel containers (UFCs). Microbial-influenced corrosion is a potential threat to long-term integrity of UFC as bisulfide (HS⁻) may be produced by microbial activities under anaerobic conditions and transported via diffusion through the HCB to reach the UFC surface, resulting in corrosion of copper. Therefore, understanding HS⁻ transport mechanisms through HCB is critical for accurate prediction of copper corrosion allowance. This study investigated HS⁻ transport behaviour through MX-80 bentonite at dry densities 1070–1615 kg m⁻³ by performing through-diffusion experiments. Following HS⁻ diffusion, bromide (Br⁻) diffusion and Raman spectroscopy analyses were performed to explore possible physical or mineralogical alterations of bentonite caused by interacting with HS⁻. In addition, accessible porosity $\left(\epsilon \right)$ was estimated using extended Archie's law. Effective diffusion coefficient of HS⁻ was found 2.5 × 10⁻¹² m² s⁻¹ and 5.0× 10⁻¹² m² s⁻¹ for dry densities 1330 and 1070 kg m⁻³, respectively. No HS⁻ breakthrough was observed for highly compacted bentonite (1535–1615 kg m⁻³) over the experimental timeframe (170 days). Raman spectroscopy results revealed that HS⁻ reacted with iron in bentonite and precipitated as mackinawite and, therefore, it was immobilized. Finally, results of this study imply that HS⁻ transport towards UFC will be highly controlled by the available iron content and dry density of the buffer material.