Euradwaste '08 - EU Bookshop - Europa

porosity outside the clay interlayer volume, and it is expected that anion repulsion will also affect
the amount and ‘geometry’ of this ‘external’ porosity accessible for anion diffusion. Cations and
neutral species (HTO), on the other hand, are able to access and diffuse in, all of the pore volume.
RTDC3 consecrated a significant effort toward increasing understanding and modelling equilibrium
mass distribution (accessible porosities) and mobility (diffusion) of anions, HTO (and cations) in
clay mineral domains at the microscopic-scale (~ m), and testing the models against experimental
data measured on a compacted pure clay mineral (montmorillonite) synthesized and characterized
specifically for project needs (LMPC).
From a theoretical and modelling perspective, the challenge is to describe and model, in a scientifically
rigorous fashion, diffusion of anions, cations and HTO molecules in compacted clay materials
as a function of material density (which affects the pore size distribution) and solution composition
(cation charge, ionic strength…). The main outcomes and advances in understanding along this line
are summarized below:
Results of molecular dynamics simulations of a montmorillonite in contact with a NaCl
solution (BRGM, AIED) were used to estimate reasonable bounds for the anion exclusion
volume, distance of water structuring and cation partitioning between the diffuse layer and
the sorbed plane in the system. The H2O data were consistent with 2 H-NMR measurements
(LAIEM) on the synthetic clay mineral. The calculated ion distributions were then used to
‘calibrate’ the electrical double layer parameters of a surface complexation model contained
in a code capable of coupling geochemical speciation and diffusion (PHREEQC2 v2.14).
This code was then used to calculate diffusion of HTO anions and cations through
compacted montmorillonite under different conditions (density, solution composition).
Comparison of model results with existing data sets show that it is able to simulate the
principal observed characteristics. The results of comparison with the experimental data
obtained in FUNMIG (see below) are not yet available.
Two new theoretical (and corresponding numerical) models of anion, cation and HTO
diffusion in compacted clay domains were developed respectively by Armines and CEA. The
distinctive feature of the Armine model is that it proposes that the hydration water associated
with cations present in clay interlayers be treated as part of the solid phase, i.e. not as a
constitutive part of overall porosity. This paradigm change allows a single value to be used
for the porosity accessible for anion, cation and HTO diffusion, with only the latter two
molecules being able to exchange mass with the pools of cation and hydration water present
in the interlayer (solid). This model, which has many similarities with that developed by
BRGM, is able to satisfactorily model data sets of anion, cation and HTO diffusion over a
wide range of clay densities.
The model developed by CEA takes a completely different approach, representing the compacted
clay in terms of an ordered arrangement of charged, non porous, several nanometrethick
rectangular entities, immersed in a continuous dielectric medium (the pore solution).
The rectangles represent the external surfaces (basal and edge), i.e. the interlayer volume is
not considered, of real clay particles. Changes in pore size distribution as a function of density
is taken into consideration by changing the particle population spacing. It is worth noting
that this is the only model approach, at this scale, which yields different values for De
perpendicular or parallel to particle orientation, i.e. this model allows introduction of diffusion
anisotropy in the clay domains. Model simulation results are generally coherent with
experimental observations of diffusion in compacted clays; e.g. De and accessible porosity
values for anions decrease with ionic strength, De for the alkaline elements increase from Na
to Cs.
314