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Morichon E.,University of Poitiers | Morichon E.,University Paris Diderot | Morichon E.,Center Regional dInnovation du Biopole | Allard T.,University Paris Diderot | And 2 more authors.
Physics and Chemistry of Minerals

Natural radiation-induced defects were identified in specimens of sudoite (Al-Mg di-trioctahedral chlorite) related to unconformity-type uranium deposits at the base of the Athabasca Group (Saskatchewan, Canada), using electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies. X-band spectra indicate the presence of a main native defect, named the As-center, whose EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g// = 2,051 and g⊥ = 2,005, and a secondary defect with apparent component g = 2,025. The study of oriented specimens shows that the main defect has its g// component perpendicular to the (ab) plane of sudoite. The As-center corresponds to an electron hole located on oxygen atoms of the structure and is likely associated with Si, according to the lack of hyperfine structure. The As-center in sudoite has EPR parameters similar to the A-center in kaolinite and dickite, and the Ai-center in illite. The saturation behavior of EPR spectra as a function of power demonstrates that native defects of sudoite are different from those known in other clays, such as kaolinite, dickite or smectite, but are similar to those of illite. The isochronal annealing data suggest that the main defect in sudoite is stable to more than 300°C. The corresponding defects characterized in sudoite may have the potential for tracing past radionuclide migration around unconformity-type uranium deposits. © Springer-Verlag 2009. Source

Myllykyla E.,VTT Technical Research Center of Finland | Tanhua-Tyrkko M.,VTT Technical Research Center of Finland | Bouchet A.,Center Regional dInnovation du Biopole
Materials Research Society Symposium Proceedings

This study aims at gaining a better understanding of the behaviour of montmorillonite in contact with different ground waters; alteration of montmorillonite and possible formation of secondary minerals. Batch experiments were conducted with purified Swy-2 montmorillonite in simulated fresh (I=0.05 M, pH 8) and saline (I=0.1 M, pH 11) waters at 25 and 60°C in anaerobic (Ar(g)) conditions. The concentrations of Al, Fe; Mg and Si were analysed from ultra-filtered solution samples with HR-ICP-MS (High Resolution Inductively Coupled Plasma Mass Spectrometry). The amount of released Si depended strongly on the experimental conditions. The Si concentrations at 60°C in the saline and fresh waters showed a difference greater than an order of magnitude. The initial purified montmorillonite and the solid materials from experiments were analysed with XRD. The analysis indicated that the nature of smectite did not change, but the experimental conditions, more or less, modified the structure of montmorillonite, e.g., in fresh waters the XRD spectra showed peaks typical of mixed layer minerals, which can refer to the presence of either randomly ordered illite/smectite or randomly ordered collapsed smectite/hydrated smectite layers. The dissolution of montmorillonite was studied also by modelling with TOUGHREACT. The experimental and modelled results were compared revealing a need to develop the model e.g. in respect of the evolution of pH. © 2012 Materials Research Society. Source

Myllykyla E.,VTT Technical Research Center of Finland | Tanhua-Tyrkko M.,VTT Technical Research Center of Finland | Bouchet A.,Center Regional dInnovation du Biopole | Tiljander M.,Geological Survey of Finland
Clay Minerals

The effects of simulant groundwater composition, pH and temperature on the dissolution and alteration of Na-and Ca-montmorillonite have been studied. Prior to the experiments, Wyoming type Na-montmorillonite, Swy-2, was purified to decrease the amount of accessory minerals. For Ca-montmorillonite experiments, the interlayer cation Na+ of purified Swy-2 was exchanged with Ca2+. The batch experiments were conducted with the purified montmorillonites in simulated fresh and saline waters at 25 C and 60 C under anaerobic conditions in an Ar atmosphere. The concentrations of Si, Al, Fe and Mg were analysed from ultra-filtered solution samples with High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) as a function of dissolution time. The pH evolution was also measured. The solid smectite phases were analysed with X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD analyses indicated that the nature of the smectite mineral did not change over 140 days. However, the experimental conditions, more or less, modified the structure (e.g. the layer stacking of montmorilllonite; the partial dissolution of the smectite), which cannot be detected by XRD but was evidenced by chemical data, and can be considered as a possible contributor to the stacking faults of the montmorillonite. The log rates (mol g-1 s -1), based on the dissolved amount of Si, varied between-10.64 and-12.13 depending on the experimental conditions. © 2013 Mineralogical Society. Source

Vinsot A.,Andra Inc | Leveau F.,Andra Inc | Bouchet A.,Center Regional dInnovation du Biopole | Arnould A.,GEOTER
Geological Society Special Publication

Deep argillaceous rocks are reducing environments. When exposed to air, reduced minerals of these rocks react with oxygen, modifying the surrounding chemical conditions. Thus, oxidation is an issue in studies about the confining properties of such rocks in the framework of geological disposal projects for radioactive waste. Previous studies in several underground research laboratories (URLs) in argillaceous rocks have shown that oxidation reactions mainly occurred in the excavation-induced fracture network surrounding the drifts. In the Callovian- Oxfordian argillaceous rock, at 2490 m in drifts from the Meuse/Haute-Marne URL, oxidized features were systematically looked for in 115 borehole cores. The concerned drifts were of various ages, from a few days to 6.5 years. After 5 months, oxidized features were encountered in numerous excavation-induced extensional fractures. In excavation-induced shear fractures, oxidized features were observed in a few borehole cores after 2 years, and they became frequent after 6 years. In all cases, the oxidized features observed were found on the fracture walls or were connected to them, and were less than 1.8 m from the drift walls. These observations about the oxidation front and its evolution over time provide insights regarding the properties of excavationinduced fractures with respect to oxygen transfer. © The Geological Society of London 2014. Source

Altmann S.,Andra Inc | Tournassat C.,Bureau de Recherches Geologiques et Minieres | Goutelard F.,CEA Saclay Nuclear Research Center | Parneix J.-C.,Center Regional dInnovation du Biopole | And 3 more authors.
Applied Geochemistry

Clay mineral-rich sedimentary formations are currently under investigation to evaluate their potential use as host formations for installation of deep underground disposal facilities for radioactive waste (e.g. Boom Clay (BE), Opalinus Clay (CH), Callovo-Oxfordian argillite (FR)). The ultimate safety of the corresponding repository concepts depends largely on the capacity of the host formation to limit the flux towards the biosphere of radionuclides (RN) contained in the waste to acceptably low levels. Data for diffusion-driven transfer in these formations shows extreme differences in the measured or modelled behaviour for various radionuclides, e.g. between halogen RN ( 36Cl, 129I) and actinides ( 238,235U, 237Np, 232Th, etc.), which result from major differences between RN of the effects on transport of two phenomena: diffusion and sorption. This paper describes recent research aimed at improving understanding of these two phenomena, focusing on the results of studies carried out during the EC Funmig IP on clayrocks from the above three formations and from the Boda formation (HU).Project results regarding phenomena governing water, cation and anion distribution and mobility in the pore volumes influenced by the negatively-charged surfaces of clay minerals show a convergence of the modelling results for behaviour at the molecular scale and descriptions based on electrical double layer models. Transport models exist which couple ion distribution relative to the clay-solution interface and differentiated diffusive characteristics. These codes are able to reproduce the main trends in behaviour observed experimentally, e.g. D e(anion) Source

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