Diversity and potential activity of microorganisms present in natural clay and bentonite: Key components of deep geological repositories for used nuclear fuel

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Agencies responsible for used nuclear fuel management have selected deep underground storage in engineered repositories in either crystalline or sedimentary rock formations as a safe, long-term solution (100,000 to 1,000,000 years) to the growing used fuel inventory. Over such long time frames, potential exists for the metabolic activities of either indigenous or introduced microbes to impact the repository integrity (e.g., by corroding fuel canisters via sulphide production, or by producing gases that could pressurize the repository and cause fractures in the clay barrier). As well, recent studies examining the suitability of 170 million year-old sedimentary deposits at Mont Terri, Switzerland, as a host rock formation revealed the presence of very low numbers of a variety of microbes, including those with potential sulphate reducing capabilities, and when a source of electron donors was provided, these numbers increased significantly. The significance of these findings is not yet certain; however, technology-transfers applicable to the Canadian deep geological repositories (DGR) concept are anticipated. The proposed research will investigate various scenarios whereby microorganisms may impact key components of DGR for used nuclear fuel, including the applied and evolutionary significance of microbial populations harboured in ancient clays like the Mont Terri formation, as well as transformation of the repository atmosphere from an oxidizing to reducing state. Model systems simulating repository conditions will help to evaluate the performance of engineered processes and conditions in terms of the growth and activity of key microorganisms, and in particular, the extent to which corrosive and metabolic intermediates are formed. The potential for interfaces between the various engineered barrier system components to provide refuges for biofilm formation with enhanced metabolic activity is also a key goal. Such knowledge is important to ensure that Canadian DGRs are engineered to mitigate microbial effects that may occur over the expected repository life cycle, and to aid in the development of predictive computer models that strengthen Canada's long-term DGR design and performance criteria.