Leveraging microbial processes to mitigate hydrogen pressure buildup in radioactive waste repositories

Nuclear energy expanded rapidly during the second half of the twentieth century, to become a major source of low-carbon electricity in many countries, including Switzerland. However, reliance on nuclear power brings with it the challenge of long-term management of radioactive waste. Safety standards require that radioactive waste be isolated from humans over timescales of up to one million years. To date, a broad scientific consensus considers deep geological repositories to be the safest option for the final disposal of this waste. Ensuring the long-term safety of such repositories requires a thorough understanding of coupled mechanical, hydraulic, chemical, and biological processes. Among these, microbial activity has received increasing attention, particularly regarding hydrogen produced by the anoxic corrosion of steel components, such as waste containers. Hydrogen may accumulate and generate overpressure, which could threaten the stability of the host rock; the rock in which the waste will be placed. Microbial hydrogen consumption acts as a gas sink, mitigating pressure buildup and potentially reducing technical constraints. This thesis investigates microbial hydrogen consumption under conditions representative of those expected in the future Swiss repository. The focus here is on microbial activity in sand-bentonite, a candidate tunnel backfill material that will gradually saturate with water from the host rock and has been shown to host hydrogenotrophic microorganisms. First, we delineated the threshold water saturation below which the backfill material can no longer support microbial activity. We quantitatively assessed hydrogen consumption rates at four levels of water saturation. Our results highlight the critical role of pore-scale water connectivity and indicate that hydrogen consumption rates strongly depend on community composition. Second, we developed a medium-scale experimental setup enabling operation at high hydrogen concentrations, continuous monitoring of total pressure evolution and highly sensitive gas composition analyses. Rapid hydrogen depletion was coupled primarily to sulfate and iron reduction, and to a lesser extent to methanogenesis. Hydrogen consumption rates were controlled by water saturation, pH, and the availability of electron acceptors. Consumption was also faster than in experiments conducted at lower hydrogen concentrations, which was associated with a higher abundance of hydrogenotrophic microorganisms. Third, we unraveled the metabolic pathways responsible for hydrogen consumption. Metagenomic analyses combined with geochemical measurements reveal a simple microbial ecosystem sustaining hydrogen oxidation involving sulfate and iron reducers. This work provides mechanistic insight into the pathways and organisms involved. Carbon metabolic pathways in non-hydrogenotrophic microorganisms enable the breakdown of complex compounds from necromass or recalcitrant carbon associated with bentonite, thereby provid