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Experimental data sets, for comparison with the blind predictions made using the theoretical
models described above, are being generated by carrying out diffusion experiments with an
anion ( 36 Cl), HTO and mono and divalent cations ( 22 Na, 45 Ca) on compacted synthetic
montmorillonite samples, as a function of ionic strength (CEA). While these measurements
are not completely finished at this time, initial results show for example that, as expected, (i)
anion exclusion increases with decreasing ionic strength and there is no impact of ionic
strength on HTO diffusion and (ii) 45 Ca diffusion is enhanced by and strongly depends on
ionic strength (due to reduced Ca 2+ sorption due to competition of Mg 2+ for ion exchange
sites).
3.2 Diffusion in mesoscopic scale (~mm) clayrock volumes
Considerable effort was invested in improving understanding of how clayrock mineral-porosity organization
can affect diffusion of mobile (non sorbing) RN since this seems to be a highly promising
approach for establishing links between diffusion properties and observed variations in rock
mineralogy. The working hypothesis, based on the observations presented in §2.3, was that the spatial
organization of the contiguous clay matrix porosity could affect (i) the value of the apparent diffusion
coefficient (Da) for non sorbing tracers by modifying diffusion path tortuosity and (ii) the
anisotropy of Da values measured in directions perpendicular or parallel to the sedimentation
planes. The study, carried out by Hydr’asa/Andra, was based on simulations of HTO diffusion in
numerical models of the 2D and 3D mineral-porosity distributions quantified in §2.3. These simulations
were carried out using the Time Domain Diffusion (TDD) method which simulates diffusion
by tracking the ‘random walk’ of anion particles in the 2D or 3D pixel grids based on the digitized
images of mineral grain organization. Each grid pixel is characterized by its porosity (a constant
value for all clay pixels, null for all others) and an isotropic Da value (for the clay pixels). The effects
of grain organization were quantified by performing simulations of diffusion, in rock volumes
having different compositions, in directions parallel and perpendicular to the sedimentation surface
plane. The results show (i) that Da perpendicular to the sedimentation surface decreases with increasing
fraction of non porous minerals and (ii) that the elongated shape of carbonate and quartz
grains and their orientation relative to the sedimentation surface introduce geometrical anisotropy in
the organization of the connected porosity at the mesoscopic scale, which in turn induces diffusion
anisotropy at a larger scale. The anisotropy of diffusion, which is observed experimentally, probably
has two components: inside and outside the clay matrix. The global diffusion coefficient is related
to the clay matrix diffusion coefficient by a geometric factor Gm which is specific to the clay
matrix geometry (this work). The clay matrix diffusion coefficient is itself related to free diffusion
of solute and a geometric factor Gcp related to clay particles (as described by the models presented
in §3.1).
3.3 Diffusion at the macroscopic scale (cm-dm)
Several premises are behind diffusion measurements made at the ~cm scale, among the most important
being (i) that they are made on samples representative of the ‘average’ properties (mineralogical
composition, porosity characteristics, etc.) of the rock unit from which they were taken and (ii)
that the measured parameter values (De, Da(perpendicular to bedding); De, Da(parallel), accessible
porosity, mineralogy, etc.) integrate, in a representative fashion, the effects on these parameters of
local variations in rock properties at smaller scales. One of the working hypotheses guiding the
RTDC3 experimental program at this scale was that the major characteristics of anion diffusion
should be coherent with, and explainable by, phenomena which were studied and modelled at the
smaller, mesoscopic scale (§3.2), in particular the role of non porous mineral grain organization in
determining diffusion anisotropy, i.e. Da(bedding parallel) > Da(bedding perpendicular), and the
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