Canada’s plan for a deep geological repository (DGR) will provide a safe, long-term storage solution for used nuclear fuel. Investigating microbial viability in Wyoming MX-80 bentonite under DGR-like conditions suggests microbial suppression in low water activities for the prevention of microbiologically influenced corrosion.

Canada is designing a deep geological repository (DGR) for long-term storage of used nuclear fuel in a multi-barrier system 500 metres belowground. Highly compacted bentonite (HCB) clay, which swells upon saturation, will encase used nuclear fuel containers and seal the DGR. An important consideration is the possibility of microbiologically influenced corrosion of the used fuel containers, particularly due to sulfate-reducing bacteria (SRB) that may be native to the bentonite or subsurface environment. Previous studies have shown that in HCB, microbial growth is suppressed when the swelling pressure is over 2 MPa and the water activity is under 0.96. Before reaching a state of equilibrium, the system will experience variability in temperature, pressure, and water activity. It is therefore essential to study microbial viability and growth in HCB under various conditions. This research investigated the effects of water activity of bentonite on microbial viability, abundance, and microbial community composition, without the influence of pressure. Bentonite clay microcosms were hydrated to water activities of 0.93, 0.96, or 0.99 with water, low or high salinity solutions, or SRB medium and were incubated in oxic or anoxic conditions. Over a period of six months, aerobic and anaerobic heterotrophs as well as SRB were cultured and enumerated, and 16S rRNA genes were quantified and sequenced. High water activities result in an increase of culturable aerobic heterotrophs for oxic conditions, while SRB abundances remain unchanged over time in all tested conditions. High salinity further suppressed microbial growth. Water-activity-dependent actinobacterial growth was observed for oxic water and SRB medium hydrated microcosms. Overall, the results demonstrate that lower water activities proposed for engineered bentonite barriers for a DGR slow microbial growth, in the absence of pressure, and these results complement ongoing parallel pressure vessel experiments that combine water activity and pressure.