Stockage des déchets radioactifs en couche géologique profonde

Le stockage en couche géologique profonde, parfois appelé enfouissement, est envisagé pour confiner les déchets nucléaires, en complément du stockage en surface dans plusieurs pays, en particulier pour les déchets de haute et moyenne activité à vie longue. Il consiste à conditionner ces déchets puis à les placer dans une formation géologique stable en interposant des barrières naturelles et artificielles entre les déchets et l'environnement. Ce mode de gestion repose sur l'hypothèse que la rétention des déchets peut atteindre une durée suffisante pour assurer leur décroissance radioactive. Le stockage des déchets nucléaires en profondeur est un sujet de polémique dans le monde entier et de nombreux mouvements et organisations écologistes luttent contre cette solution qu'ils jugent peu sûre, coûteuse et inutile. Contexte Les déchets radioactifs sont actuellement produits dans différents pays. La gestion de ces différents types de déchets repose sur leurs caractéristique

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

Niels Burzan

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.

EPFL2021

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

Biogeochemical Cycling by a Low-Diversity Microbial Community in Deep Groundwater

Louise Julia Balmer, Emma Bell, Rizlan Bernier-Latmani, Manon Frutschi, Guillaume Sommer

Olkiluoto, an island on the south-west coast of Finland, will host a deep geological repository for the storage of spent nuclear fuel. Microbially induced corrosion from the generation of sulphide is therefore a concern as it could potentially compromise the longevity of the copper waste canisters. Groundwater at Olkiluoto is geochemically stratified with depth and elevated concentrations of sulphide are observed when sulphate-rich and methane-rich groundwaters mix. Particularly high sulphide is observed in methane-rich groundwater from a fracture at 530.6 mbsl, where mixing with sulphaterich groundwater occurred as the result of an open drill hole connecting two different fractures at different depths. To determine the electron donors fuelling sulphidogenesis, we combined geochemical, isotopic, metagenomic and metaproteomic analyses. This revealed a low diversity microbial community fuelled by hydrogen and organic carbon. Sulphur and carbon isotopes of sulphate and dissolved inorganic carbon, respectively, confirmed that sulphate reduction was ongoing and that CO2 came from the degradation of organic matter. The results demonstrate the impact of introducing sulphate to a methane-rich groundwater with limited electron acceptors and provide insight into extant metabolisms in the terrestrial subsurface.