Disposal facilities in deep geological formations are considered to be a possible solution for long-term management of high-level nuclear waste (HLW). The design of the repository generally consists of a multiple-barrier system including Fe-based canisters and a clay backfill material. The Fe-clay system will undergo a thermal gradient in time and space, the heat source being the HLW inside the canisters. In the present paper, the effect of a thermal gradient in space on Fe-smectite interactions was investigated. For this purpose, a tube-in-tube experimental device was developed and an 80–300ºC thermal gradient was applied to a mixture of MX80 bentonite, metallic Fe (powder and plate), magnetite, and fluid over periods of 1 to 10 months. Transformed and newly formed clay minerals were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. The main mineralogical transformations were similar to those described for batch experiments: smectite was destabilized into an Fe-enriched trioctahedral smectite and Fe-serpentine or chlorite as a function of the experimental conditions. Newly formed clay was observed all along the walls of the gold tube. Their crystal chemistry was clearly different from the clays observed in the hot and cold part of the tubes. The thermal diffusion of elements was also observed, especially that of Mg, which migrated toward the hottest parts of the tubes. In the end, the thermal gradient affected the redox equilibria; more reduced conditions were observed in the hotter parts of the tubes.

MX-80 is a bentonite clay from the Clay Spur bed, Wyoming, USA, consisting of ~80 wt.% Na-rich montmorillonite and is a leading candidate material for use in engineered barriers within underground nuclear waste disposal sites in Switzerland and Sweden. The diffusive invasion of groundwater into the engineered barriers is likely to cause the dry high-density bentonite to swell and evolve into a less-dense, water-rich gel during re-submergence of the shafts and galleries. Data on H20 diffusivity within water-rich bentonite gels are therefore essential to the safe design of the engineered barriers. To address this need, the self-diffusion coefficient, D, of 1H2O molecules in water-rich MX-80 bentonite gels was measured using the pulsed-gradient spin-echo method of 1H nuclear magnetic resonance spectroscopy at 0.47 T; D was measured over a wide range of temperatures (20.6–70.1°C) and of bentonite volume fractions (0 to 17.2 vol.%). The results showed that D increased markedly to values of bulk water self-diffusivity as the bentonite volume fraction decreased towards 0 vol.%. The activation energy of the diffusion process for the gels was approximately equal to that for bulk water. As a result, the normalized H20 self-diffusivity, D/D0, was approximately independent of temperature, where D0 is D in bulk water. The D/D0 data for ≤6.39 vol.% bentonite fraction were successfully fitted to a porous clay gel model that involves unbound water molecules that diffuse in the pore space by avoiding randomly placed discoidal obstacles (clay particles) of a high aspect ratio. The D/D0 data for MX-80 were similar to data for SWy-2 (Wyoming bentonite from the Newcastle formation) and Kunigel-Vl (bentonite from Yamagata, Japan), suggesting that all three bentonites are equally suitable candidate materials, in terms of water diffusivity performance, for use within engineered barriers.

This paper describes the swelling properties of two highly compacted clays, natural, untreated Wyoming montmorillonite (MX80) and Fourges smectite (FoCa7), saturated with Na and Ca, respectively.

The initially compacted samples were hydrated by subjecting them to different suction pressures in a range between 100 MPa and 1 kPa. For each equilibrium state, the volume change (swelling) and water content (hydration) were measured. The samples were then studied by X-ray diffraction using a transmission device to determine interlayer distance and particle size, in order to clarify both the swelling and hydration mechanisms. The distances between clay layers ranged between 10 and 21.6 Å , i.e. corresponding to between 0 and 4 water layers. Upon hydration, the particle size decreased from 350 and 100 clay layers per particle to 10 layers per particle when the suction pressure decreased from 100 MPa to 1 kPa for MX80 and FoCa7, respectively. The first swelling stage is described as being an insertion of water molecules between the layers. Then a division of the initial particles into particles of smaller size with increasingly large inter-particle distances was observed. Observations by transmission electronic microscopy confirmed these results.