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cluding research institutes, laboratories, SME, national radwaste management agencies and four
Associated Groups (Hydr’asa(FR), La Trobe University(AUS), CEREGE(FR), UnivAvignon(FR)).
2 Characterizing and understanding clayrock properties influencing RN migration
Clayrock composition and structure largely govern the migration characteristics for any given RN
species. This rather blunt affirmation is in fact the working hypothesis for a significant part of the
research carried out in RTDC3, which is why it merits explanation, in particular in relation to the
two situations studied in detail in RTDC3: diffusion-driven transport of anionic RN species and retardation-by-sorption
of cationic RN species. First, what do we mean by composition and structure?
Composition includes both the mineralogical (and organic) phases making up the rock solid matrix
and, most importantly, the speciation of the pore solution and contacting mineral surfaces. Pore solution
and mineral surface speciation will largely determine RN dissolved speciation (e.g. predominance
of anionic or cationic forms), solid-solution partitioning of RN mass (Kd) and the intensity of
electrostatic field effects on the solution volume accessible to anionic RN. As for structure, it refers
to the organization, geometry and dimensions of the connected porosity of a given clayrock, and its
relation to contacting minerals. Porosity structure, taken together with the electrostatic field effects
on anions mentioned above, will determine the accessible porosity and diffusion path ‘tortuosity’
for cationic and anionic RN species. In addition, since current knowledge indicates that the permanently
negatively charged swelling clay minerals present in clayrocks play a key role in determining
RN migration behaviour, the structure and composition of the porosity associated with the clay
mineral fraction is expected to be of prime importance. In addition, clayrock formation databases
(1, 2, 3) generally show that values measured for a given composition or structure parameter vary
for rock samples taken from different positions within the formation, i.e. the formation is not an
homogeneous entity as regards parameters which might affect RN migration.
Research carried out in RTDC3 focused on enhancing understanding of clayrock structure and composition
at the various scales and spatial resolutions which will be needed for interpreting, integrating
and up-scaling the results of studies on RN diffusion and sorption described in subsequent sections,
i.e.
at the formation, i.e. Geological Barrier System (GBS), scale (~10 2 m) with a resolution of
~10 -1 m,
at the ‘macroscopic’ scale (10 -1 to 10 -2 m), typical of that associated with lab and in-situ
determinations of RN Kd (or R) and De parameters, with resolutions ranging down to ~ 10 -5
m,
at the ‘mesoscopic’ scale (~ 10 -3 to 10 -4 m), characteristic of that of diffusion profiles for
high Kd RN (cf. §4.2), with resolutions down to 10 -6 m.
All scales were studied on the COx formation in order to provide a common ‘safety case’ context
for inter-relating and up-scaling study results. Clayrock samples from all four clayrock formations
(Opalinus clay, Boom clay, Callovo-Oxfordian, Boda claystone (HU)) were also characterized in
terms of mineralogy, structure, porewater composition and water states at the macroscopic scale in
order to identify key common characteristics and essential differences likely to impact RN migration
(CIEMAT, ERM).
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