Why should we care about microorganisms in the bentonite back fill material for the storage of high-level radioactive waste in deep geological repositories?

Every year 12,000 metric tonnes of high-level radioactive waste (HLW) are produced worldwide. For the long-term storage of this highly radiotoxic waste, a deep geological disposal by using multiple barriers is favored. Bentonite is proposed as a potential material for sealing the space between the canister containing the HLW and the surrounding host rock. In order to investigate the microbial diversity and metabolic activity of naturally occurring microorganisms as well as their time-dependent evolution, we conducted anaerobic microcosm experiments containing bentonite and a synthetic Opalinus Clay pore water solution. During the one-year incubation at 30 and 60 °C, lactate- or H2-stimulated microcosms at 30 °C showed the dominance and activity of strictly anaerobic, sulfate-reducing and spore-forming microorganisms. The subsequent generation of hydrogen sulfide gas in the respective set ups, led to the formation of fractures and iron-sulfur precipitations. In microcosms that incubated at 60 °C, thermophilic bacteria dominated, independent from the availability of substrates. In the respective microcosms, no significant metabolic activity was detected and there was no change in the analyzed bio-geochemical parameters. Our results show that indigenous microorganisms evolve in a temperature- and substrate-dependent manner. Potentially formed metabolites could affect the dissolution behavior of minerals and ions within the bentonite as well as the corrosion process of the canister material and require further investigations.