Processes of cation migration in clayrocks: Final Scientific Report of the CatClay European Project
Rapport final du projet européen CatClay sur les processus de migration des cations dans les roches argileuses indurées
Résumé
In the framework of the feasibility studies on the radioactive waste disposal in deep argillaceous formations, it is
now well established that the transport properties of solutes in clay rocks, i.e. parameter values for Fick’s law, are mainly
governed by the negatively charged clay mineral surface. While a good understanding of the diffusive behaviour of non-reactive
anionic and neutral species is now achieved, much effort has to be placed on improving understanding of coupled
sorption/diffusion phenomena for sorbing cations. Indeed, several cations known to form highly stable surface complexes with
sites on mineral surfaces migrate more deeply into clay rock than expected. Therefore, the overall objective of the EC CatClay
project is to address this issue, using a ‘bottom-up’ approach, in which simpler, analogous systems (here a compacted clay,
‘pure’ illite) are experimentally studied and modelled, and then the transferability of these results to more complex materials, i.e.
the clay rocks under consideration in France, Switzerland and Belgium for hosting radioactive waste disposal facilities, is
verified. The cations of interest were chosen for covering a representative range of cations families: from a moderately sorbing
cation, the strontium, to three strongly sorbing cations, Co(II), Zn(II) and Eu(III). For the 4 years of this project, much effort was
devoted to developing and applying specific experimental methods needed for acquiring the high precision, reliable data needed
to test the alternative hypotheses represented by different conceptual-numerical models. The enhanced diffusion of the sorbing
cations of interest was confirmed both in the simpler analogous illite system for Sr2+, Co(II) and Zn(II), but also in the natural
clay rocks, except for Eu(III). First modelling approach including diffusion in the diffuse double layer (DDL) promisingly
succeeded in reproducing the experimental data under the various conditions both in illite and clay rocks, even though some
assumptions made have to be verified. In parallel, actual 3D geometrical pore size distributions of compacted illite, and in less
extent, clay rock samples, were successfully determined by combining TEM and FIB-nt analyses on materials maintained in a
water-like saturation state by means of an extensive impregnation step. Based on this spatial distribution of pores, first numerical
diffusion experiments were carried at the pore scale through virtual illite, enabling a better understanding of how transfer
pathways are organized in the porous media. Finally, the EC CatClay project allowed a better understanding of the migration of
strongly sorbing tracers through low permeability ‘clay rock’ formations, increasing confidence in our capacity to demonstrate
that the models used to predict radionuclide migration through these rocks are scientifically sound.
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