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Iron-clay reactivity in radioactive waste disposal - Impacts of bacterial activities and heterogeneities

Camille Chautard has defended her thesis on 4th December 2013 in Paris.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/DSU/SSIAD/BERIS

Authors > CHAUTARD Camille

Publication Date > 04/12/2013

Summary

This study focuses on the interactions between two materials that may be introduced in a geological disposal of radioactive waste: metallic materials such as the high-level waste overpack, and clay materials such as the clay host rock. Indeed, the interactions between these two materials in such conditions could induce a change of their initial confinement properties. This work aimed at determining the influence of heterogeneities (technological gaps and fractures) and bacterial activities on these interactions, in terms of evolution of chemical and hydraulic properties of clayey materials. To this end, two percolation cells have been conducted during 13 months: the first one with two bacteria (SRB, IRB), the second one without bacteria. These experiments, carried out at 60°C, involved circulating synthetic water representative of the Tournemire pore water through iron powder and through Toarcian artificially cracked argillite from Tournemire. An iron rod was also placed into the argillite. Thus, solid characterizations (SEM, SEM/EDS, Raman, XRD, X-ray tomography) allowed the study of both interfaces: the iron powder/argillite interface and the iron rod/argillite interface.

The water probably circulated into the crack during the entire test, which was confirmed by reactive transport modeling with the HYTEC reactive transport code. However, no secondary phase was identified in the crack. In addition, bacteria survival in the biotic cell was confirmed during the experiment by monitoring their population and by analyzing their genetic diversity at the end of the experiment. A strong decrease in sulfate concentration was measured in the output, which confirms the SRB activity. Solid characterization conducted at the end of the experiments have highlighted, with and without bacteria, the occurrence of magnetite and chukanovite in the iron powder, the latter being mainly located close to the argillite interface. In the argillite, a Feenriched zone (10 μm) was identified. The mean corrosion rate was estimated at 0.2 μm/y (lower bound). At the iron rod/argillite interface, two corrosion facies were observed. The first, identified in both cells, is mainly constituted of chukanovite in the inner layer and siderite in the outer layer. Extent of the argillite perturbation reaches about 30 μm. The second, only observed with bacteria, highlights the presence of iron sulfide precipitation (mackinawite) and localized corrosion patterns. The mean corrosion rate was estimated at 12 μm/y with and without bacteria. Finally, HYTEC simulations have enabled us to better understand the observed biogeochemical processes, such as the pH effect, and to better quantify some key kinetic parameters.

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