Intergranular pore space evolution in MX80 bentonite during a long-term experiment

Abstract

Focused ion beam nanotomography (FIB-nt) was applied to MX80 bentonite samples from the long-term Alternative Buffer Material (ABM) experiment in order to study the evolution of the intergranular pore space under similar condition that is supposed to prevail in repositories of nuclear waste. The applied high-resolution imaging method revealed the presence of two different types of pore filler. The first type is related to corrosion of iron and is represented by newly formed heavy minerals. Extensive formation of heavy minerals occurred only near the iron parts of the experimental set up. Based on comparison with other studies, the second filler type was interpreted as clay-gel that was likely formed during water uptake and swelling. A large fraction of the initial pore space was filled with such a clay gel. By attributing filled pores to the present open porosity, the initial intergranular porosity (radii N 10 nm) of the starting material was in the range of 4.3-4.6 vol.%, which was reduced to b1 vol.% during the experiment. A finite scaling approach was applied to the initial pore space (i.e. pores with radii N 10 nm), which yielded percolation thresholds with critical porosities ϕ in the range of 3-19 vol.%. Thus, the residual open porosity was far below the percolation threshold. The initial porosity of one sample was above the percolation threshold, but also in this material percolation was restricted to one spatial direction. This indicated anisotropy with respect to percolation. The formation of a claygel and heavy minerals led to a decrease in intergranular porosity, which in turn affected connectivity of the pore network. Using results from pore-network modelling in combination with percolation theory illustrates that a minor reduction of porosity led to a substantial decrease in pore connectivity. Depending on water saturation within the observed intergranular pore space, air permeability decreases exponentially over three to four orders of magnitude within a narrow porosity range of about 1 vol.%. Based on observations and calculations, gas transport along the intergranular pore space of MX80 bentonite from the ABM experiment is not considered as a possible scenario and can reasonably be excluded.

Long-term diffusion of U(VI) in bentonite: Dependence on density

“…The electrolyte is enclosed within this network. Compared to gels formed in clay suspensions, the clay gel formed in the interparticle space has a higher clay concentration and lack of free water (Keller et al, 2015). Due to the filling of the pore space with clay gel, the interparticle porosity of the clay decreases.…”

“…However, long-term mineral alteration processes in the porous medium, leading to changes in porosity, could not be excluded. Keller et al (2015) investigated the evolution of the interparticle pore space of MX-80 used in a long-term (over two years) part-time heated (about one year) experiment. They observed a formation of a so-called clay-gel or colloidal gel in the interparticle pore space.…”

Advances in Focused Ion Beam Tomography for Three-Dimensional Characterization in Materials Science

“…This paragraph summarizes the papers that have applied FIB-SEM tomograp the nuclear energy field. For example, Keller applied this technique to MX80 ben samples to study the evolution of the intergranular pores under conditions similar to found in nuclear waste repositories [201], while Hemes used a combination of micr BIB-SEM, and FIB-SEM tomography to reconstruct the Oligocene age Boom clay, wh considered to be a potential host material for radioactive waste disposal in Belgium Bulk plutonium and uranium, as well as the distribution of plutonium oxide parti Regarding the earth sciences, other works propose the reconstruction of microbialites [189], clays [18,[190][191][192], zeolites [193,194], Fe-rich olivine [195], dolomites [48,196], coals [197], soils [198], and even samples of urban dust [199]. In addition, Zhou et al investigated the formation of Au nanoparticles in porous low-Si magnetite by analyzing nanoscale structure and crystallography [200].…”

“…This paragraph summarizes the papers that have applied FIB-SEM tomography in the nuclear energy field. For example, Keller applied this technique to MX80 bentonite samples to study the evolution of the intergranular pores under conditions similar to those found in nuclear waste repositories [201], while Hemes used a combination of micro-CT, BIB-SEM, and FIB-SEM tomography to reconstruct the Oligocene age Boom clay, which is considered to be a potential host material for radioactive waste disposal in Belgium [202]. Bulk plutonium and uranium, as well as the distribution of plutonium oxide particles in the plutonium oxalate precipitates and UO 2 bubbles produced in high burnup, were also analyzed [203][204][205].…”

Pore Scale Simulation of Rheology Properties on Residence Time of Polymer Hydrogel and Hydraulic Conductivity of Bentonite Polymer Composite Geosynthetic Clay Liners

“…The typical thickness of GCLs is usually 7 mm, with granular diameters ranging from 0.002 mm to 1.5 mm [36]. After hydration, the intergranular pores between bentonite granules typically range from 0.001 mm to 0.1 mm [37][38][39][40]. Therefore, based on the typical GCL experiments, a square domain with a 7 mm × 7 mm geometry containing the same simple circular NaB granules and polymer hydrogel was assumed (see Figure 2a).…”