Growth and viability of microorganisms in bentonite and their potential activity in deep geological repository environments

Radioactive waste is among the most dangerous anthropogenic waste and its safe disposal is a challenging multi-generational task. Many nations have sought for the peaceful application of nuclear fission for the reliable production of electric energy to power their societies. The increasing volumes of radioactive waste generated are, however, proving an increasingly urgent problem to tackle. Deep geological disposal is deemed our best option for the safe long-term disposal of radioactive waste. Different host rocks are being considered for deep geological disposal, depending on the nation's geology. Switzerland favours the Opalinus Clay Formation due to its very low hydraulic conductivity and good radionuclide retention capabilities. Microorganisms in the deep subsurface are active and must be considered as their activity will influence a deep geological repository for radioactive waste. On one hand, the microorganism's activity represents a safety concern due to the possibility of enhanced corrosion, when considering the metallic waste canister encapsulating spent fuel from nuclear power plants. In other cases, however, they can contribute to the safety of a deep geological repository, when considering the gas balance of a Swiss low- and intermediate-level repository. Anoxic corrosion of metallic radioactive waste poses risks to the structural integrity of the host rock. Harnessing the activity of naturally occurring microorganisms at a safe distance from the waste itself can result in the net reduction of gases and thus their activity can be viewed as beneficial and worth exploring. In this thesis, both the positive and negative aspects of the microbial presence are explored. Wyoming bentonite is a back-fill material and an important engineered barrier system for spent nuclear fuel and high-level waste. Within the framework of an in-situ corrosion experiment, the presence and growth of microorganisms in Wyoming bentonite are investigated. The results show that the activity of corrosion enhancing sulfate-reducing bacteria in Wyoming bentonite is inhibited during the critical bentonite saturation phase but other microorganisms were able to grow. The second in-situ research project explores the potential implementation of a microbial gas sink, a natural biofilm composed of hydrogen-oxidizing sulfate-reducing bacteria and methanogenic archaea. Indeed, under the right conditions, a microbial gas sink can fulfil its intended design purposes and consume all hydrogen-gas evolving from the anoxic corrosion of the waste. This thesis contributes to our understanding of the near-field processes of a Swiss radioactive waste repository within the Opalinus Clay Formation through the contribution of experimental insight obtained under in-situ conditions.

H2-fuelled microbial metabolism in Opalinus Clay

Rizlan Bernier-Latmani, Aislinn Ann Boylan, Niels Burzan, Luca Loreggian, Carla Perez Mon

In Switzerland, the Opalinus Clay formation is considered the most likely host rock for a deep geological repository for nuclear waste. In deep geological repositories, H2 is expected to be the most abundant gas formed from the degradation of waste and from metal corrosion. The microbial community present in Opalinus Clay is capable of utilizing H2 as an electron donor and sulfate as an electron acceptor to produce hydrogen sulfide. This could be problematic due to its potential for increasing the corrosion of metal waste canisters containing radioactive waste, however, the possible impacts of these processes on the clay rock have not been fully investigated. In this study, a series of microcosm experiments were set-up containing Opalinus Clay and porewater from the Mont Terri underground research laboratory (Switzerland) as an inoculum. Uninoculated microcosms were established to investigate abiotic processes. In the presence of clay, a higher aqueous sulfate concentration was detected than in those with only porewater present and this concentration decreased over time in the inoculated experiments. However, there was no evidence of hydrogen sulfide production in the aqueous phase. In all experiments with clay, there was an increase in aqueous Fe2+ concentrations with the highest concentrations found in uninoculated experiments. The sulfur speciation of the Opalinus Clay was analysed and the results of the inoculated sample suggested that hydrogen sulfide reacted with Fe2+, precipitating iron sulfide minerals. After the incubation period, the microbial community was dominated by the sulfate-reducing Desulfobulbaceae family. The study suggests that H2-fuelled, microbially-mediated sulfate reduction can affect the mineral composition within the Opalinus Clay due to the precipitation of iron sulfide minerals. These precipitation reactions may enhance the long-term integrity of the repository by removing corrosive hydrogen sulfide from solution when sufficient Fe2+ is available and so protecting the canisters containing the nuclear waste.

2019

Fifteen years of microbiological investigation in Opalinus Clay at the Mont Terri rock laboratory (Switzerland)

Alexandre Bagnoud, Rizlan Bernier-Latmani

Microbiological studies related to the geological disposal of radioactive waste have been conducted at the Mont Terri rock laboratory in Opalinus Clay, a potential host rock for a deep geologic repository, since 2002. The metabolic potential of microorganisms and their response to excavation-induced effects have been investigated in undisturbed and disturbed claystone cores and in pore-(borehole) water. Results from nearly 15 years of research at the Mont Terri rock laboratory have shown that microorganisms can potentially affect the environment of a repository by influencing redox conditions, metal corrosion and gas production and consumption under favourable conditions. However, the activity of microorganisms in undisturbed Opalinus Clay is limited by the very low porosity, the low water activity, and the largely recalcitrant nature of organic matter in the claystone formation. The presence of microorganisms in numerous experiments at the Mont Terri rock laboratory has suggested that excavation activities and perturbation of the host rock combined with additional contamination during the installation of experiments in boreholes create favourable conditions for microbial activity by providing increased space, water and substrates. Thus effects resulting from microbial activity might be expected in the proximity of a geological repository i.e., in the excavation damaged zone, the engineered barriers, and first containments (the containers).

Springer Basel Ag2017

Hydro-mechanical characterization of gas shales and Opalinus Clay shale in partially saturated conditions

Alberto Minardi

An experimental study on the behaviour of shales in partially water saturated conditions is presented in this thesis. The Opalinus Clay shale and gas shales from different unconventional reservoirs have been studied. Opalinus Clay is currently considered as a host rock for the disposal of radioactive waste in Switzerland; the importance of partial saturation is fundamental in predicting the short-term behaviour of the host rock during the excavation-resaturation phases, and the long-term behaviour during the propagation of gas generated by the waste. Shale gas reservoirs are partially saturated in their in-situ state due to the simultaneous presence of gas and liquids; these hydraulic conditions influence the mechanical behaviour during the drilling, fracturing, and extraction stages. The analysis of the geomaterials' behaviour considering partial saturation is a well-established practice in Soil Mechanics. However, in the case of shales several challenges arise. These challenges are mainly related to their features, which make most of the experimental and theoretical frameworks adopted for soils not suitable for providing a comprehensive understanding of the behaviour of shales. The aim of this thesis is to improve upon the existing understanding of the behaviour of partially saturated gas shales and the Opalinus Clay shale and to provide a quantitative assessment of the impact of such conditions on their mechanical behaviour. A key part of this work is dedicated to the establishment of experimental methodologies and testing protocols to highlight the most important features of the response of the tested materials under partially saturated conditions. The high water retention capacity of shales necessitates the use of the vapour equilibrium technique; this experimental technique allows for the imposition of wetting/drying processes on the tested materials and introduces suction as a key parameter when performing the hydro-mechanical characterization in partially saturated conditions. Three different testing set-ups have been developed to investigate the hydro-mechanical response of the tested shales. The obtained experimental results demonstrate that the considering of partial saturation is fundamental for a reliable and comprehensive hydro-mechanical characterization of the tested shales; despite the different origins of these materials, the main outcomes show a very similar impact of wetting and drying processes on the volumetric response and the mechanical properties. The volumetric response observed in the tests performed in free stress conditions is characterized by significant anisotropy and irreversibility. However, when a mechanical stress is applied to simulate in-situ conditions, a less pronounced volumetric response is systematically observed. The elastic response is also highly dependent on the hydraulic state of the tested materials; in particular, significant reductions (up to 50%) of Young's modulus and compressive strength are observed during wetting processes. Gas injection is also studied in the Opalinus Clay to better understand the dominant flow mechanisms and to assess the possible impact of partial saturation on the movement of gas in the host rock. Experimental results obtained with a testing device specifically developed for this purpose indicate the impact of gas injection on the deformation state of the material; this aspect clearly highlights the involved hydro-mechanical coupling.