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

Area of Science:

• Geochemistry
• Materials Science
• Nuclear Engineering

Background:

• Canada's deep geological repository (DGR) uses highly compacted bentonite (HCB) to shield used fuel containers (UFCs).
• Microbial production of bisulfide (HS-) poses a risk of copper corrosion on UFCs via diffusion through HCB.
• Accurate prediction of HS- transport is crucial for assessing UFC integrity and repository safety.

Purpose of the Study:

• To investigate the transport behavior of bisulfide (HS-) through MX-80 bentonite at various dry densities.
• To explore potential physical or mineralogical alterations in bentonite due to HS- interaction.
• To determine the effective diffusion coefficient of HS- and assess its immobilization within the bentonite matrix.

Main Methods:

• Through-diffusion experiments were conducted using MX-80 bentonite at dry densities ranging from 1070 to 1615 kg/m³.
• Post-diffusion analyses included bromide (Br-) diffusion tests and Raman spectroscopy to examine bentonite alterations.
• Accessible porosity (ε) was estimated using the extended Archie's law.

Main Results:

• Effective diffusion coefficients for HS- were determined as 2.5 × 10⁻¹² m²/s (1330 kg/m³) and 5.0 × 10⁻¹² m²/s (1070 kg/m³).
• No HS- breakthrough was observed in highly compacted bentonite (1535-1615 kg/m³) within 170 days.
• Raman spectroscopy indicated HS- immobilization through reaction with iron in bentonite, forming mackinawite.

Conclusions:

• HS- transport in HCB is significantly limited by the bentonite's dry density and its iron content.
• The immobilization of HS- by iron in bentonite effectively mitigates the risk of copper corrosion on UFCs.
• These findings are critical for validating the long-term safety and performance of deep geological repositories.