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Abstract: In the context of the safety assessment of radioactive waste geological disposal in clayey formations, the evolution in time and space of clayey and metallic materials is of concern regarding their containment properties. Bacteria may induce localized corrosion processes and clay mineral alteration. Moreover, the presence of heterogeneities may favor preferential pathways or higher reactivity. Thus, the objective of the present work is to assess the influence of bacteria and heterogeneities on iron-clay reactivity. Two percolation cells are conducted in the presence of iron and argillite involving an artificial crack in biotic or abiotic conditions at 60°C during one year. Two bacteria have been selected: a Sulphate Reducing Bacteria (SRB) and an Iron Reducing Bacteria (IRB). The reactivity of those cells is evidenced by the ongoing monitoring of water composition. First results clearly show that bacteria are able to survive in the biotic experiment. Moreover, the decrease of the flow rate observed in each cell could be explained by the clogging of the crack. [Copyright &y& Elsevier]

Abstract: Mineralogical and petrographical investigations of a borehole sealed by concrete (“engineered analogue” of concrete/clay interactions) overcored at the Tournemire site offer the possibility to perform reactive transport modeling over a timescale of 15 years, which is significantly longer than standard durations of laboratory experiments. The HYTEC code was used to simulate mineralogical evolutions in the Tournemire argillite matrix and along small fractures, as well as in concrete, assuming either thermodynamic equilibrium approach or kinetic control coupled to diffusive transfer. The mineralogical profiles calculated in thermodynamic equilibrium and with kinetics are very similar in the concrete. In contrast, kinetics of dissolution and precipitation reactions is found to be necessary for a better reproduction of the experimental observations in the argillite samples compared to thermodynamic equilibrium calculations, in particular with respect to the extent of the alkaline perturbation. The major effects observed when considering kinetics in the calculations are to smooth the sharpness and intensity of the mineralogical transformation fronts, these fronts being predicted to occur deeper inside the argillite matrix (<1cm) and the fractures (⩽2cm), as well as to lessen clogging of pores within the argillite. At 15°C, the secondary mineral sequence simulated from the concrete to the argillite is: CSH/ettringite/carbonates at the concrete/argillite interface, followed by a zone of neoformation of clay-like phases and calcite, and a last zone with precipitation of calcite. Main discrepancies between the modeling results and the experimental observations are the precipitation of muscovite instead of K-feldspar and the neoformation of zeolites. Calculations also correctly predict that carbonates form at the concrete/argillite interface, which most probably result from concrete alteration and follow Ostwald’s step rule of mineral precipitation with a predominance of metastable vaterite over calcite. [Copyright &y& Elsevier]

Abstract: The presence of indigenous microorganisms in deep clayey geological formations raises the issue, regarding radioactive waste geological disposal, of the influence of bacterial activity on the confinement properties of materials, including the clayey host rock. Iron-reducing bacteria (IRB) activity is assessed in batch experiments in the presence of argillite samples from an in situ experimental laboratory at Tournemire (Aveyron, France). The results show the availability of structural Fe(III) from the clay minerals for biochemical reactions (bioreduction). In laboratory conditions, a significant impact of IRB on the alteration of clay minerals, mainly the illite–smectite mixed layer (I–Sm), is indeed observed. Such reactions may locally modify the physicochemical conditions and the stability of clay minerals (essentially smectites) prevailing in such deep facilities. More generally, bacterial activity could play an important role on clay mineral alteration and in situ experiments need to be performed in order to quantify the reaction rates and show the representative of these phenomena in the framework of the safety assessment of waste disposal. [Copyright &y& Elsevier]