Microbiology of pre-saturated gap fill bentonite compacted in pressure vessels

Abstract

Deep geologic repositories (DGRs) are the planned long-term management and disposal approach for used nuclear fuel in many countries. Designs include the use of multiple engineered barriers, including used fuel containers, bentonite buffer material, and the surrounding host rock. In Canada’s design, used fuel containers (UFCs) will be encased in highly compacted bentonite blocks and placed at depths of 650-800 m below ground surface in emplacement rooms excavated from crystalline rock, with spaces between bentonite blocks and the host rock filled with a granular bentonite (i.e., “gap fill material”). Past research has shown that microorganisms exist in as-received bentonites, some of which could negatively influence the longevity of a DGR, for example through microbially influenced corrosion of copper-coated UFCs particularly by sulfate-reducing bacteria (SRB). Previous research has focused on the use of pump-saturated pressure vessels to simulate early stages of groundwater infiltration in a DGR. Results showed suppression of microbial abundances in compacted bentonite after a relatively slow saturation process is completed and high swelling pressures have been established. To examine post-saturation microbial suppression, while minimizing changes that occur during slow saturation, pressure vessels were used to rapidly compress pre-wetted gap fill material to varying dry densities under oxic or anoxic conditions. This approach targeted nominal 100% saturation prior to incubation of the bentonite “plugs” for periods of up to three months. Following incubation, bentonite plugs were analyzed using cultivation and 16S rRNA gene sequencing. Results revealed decreasing abundances of culturable aerobic heterotrophs with increasing bentonite dry density. Relative to the as-received bentonite starting material, no significant increases in the abundance of culturable microorganisms were detected for the highest dry density tested (1.45 g cm-3). In contrast, all lower dry densities tested showed significantly greater abundances of culturable aerobic heterotrophs compared to as-received bentonite. These samples were associated with high relative abundances of three Bacillus-associated ASVs that were mostly undetected in 1.45 g cm-3 samples. Both cultivation and sequencing results demonstrated that a bentonite dry density of 1.45 g cm-3 limited detectable increases in culturable abundances under the tested conditions, which is a target average dry density under consideration for repository gap fill material. No significant increases in SRB counts were detected by cultivation methods for any conditions tested, and 16S rRNA gene profiles also did not reveal increases in taxa affiliated with putative SRB. This research also presents a rapid saturation and compaction method for studying microbial suppression in bentonite, while minimizing the influence of slow saturation effects, allowing for rapid testing of post-saturation microbial processes.