Euradwaste '08 - EU Bookshop - Europa

The links between pore apertures and mineralogy were studied combining PMMA method and
autoradiography with electron microscopy (FESEM/EDAX). Heterogeneity, anisotropy and connective
pore-network and heterogeneities were identified as a support of transport experiments with
solutes and/or colloids (WP 4.3 and WP 4.4).
Positron emission tomography (PET) studies were performed to analyse the water flow distribution
and colloid transport in a crystalline rock core from Äspö [14]. This non - destructive technique is
applied for the direct 3D visualisation of solute transport using PET tracers ( 18 F, 124 I...). The transport
paths through the fracture were observed to be modulated by the flow rate and, at localised
sites, matrix diffusion was observed [15]. The applicability of PET measurements for investigations
of the spatial distribution of transport processes of both dissolved components and colloids in granite
was demonstrated.
Geometrical description (3D) of the Äspö fractured core was also obtained by X-ray computer micro-tomography
(XTC); using this information advanced simulation of the fluid flow can be done.
A new experimental approach for non-destructive spatially resolved studies of mapping the mineral
composition and the volumetric microstructure of the host rock samples was developed, combining
3D dual-energy cone-beam tomography and X-ray fluorescence [16].
Matrix diffusion.
Matrix diffusion (MD) is considered a very important retardation process in crystalline rock above
all for not sorbing elements. The state of the art of this process was well described in RETROCK
and its theoretical bases in [17]. The effectiveness of matrix diffusion as retardation mechanism depends
on the penetration depth into the rock from the water conducting zones, and it is very dependent
on the porosity of the rock, the flow rate, RN diffusivity as well as on the flow-wetted surface.
To assess its relevance as retention mechanism it is necessary to assess the role of diffusion
against advection, if the extension of the RN diffusion within the matrix is limited or not and if the
pore system is stable over time.
One of the main recognised problems related to diffusion studies is the difficulty of obtaining experimental
data partly because of disturbances and artefacts that may exist in laboratory samples.
Additionally, diffusion lengths are extremely short (in experimental time span of months - years)
above all for highly sorbing radionuclide; thus available data are very scarce. Another process of
interest is the diffusion of anions, which can be affected by anionic exclusion, because PA calculations
showed that doses are mainly controlled by anionic species 129 I - and 36 Cl - . To demonstrate the
role of MD on the retention of these non-sorbing radionuclides is a key point.
One of the final goals of RTDC4 studies is to analyse the role of heterogeneities for the matrix diffusion
process and to develop models that explicitly take into account the heterogeneity of the rock
matrix. In most of the models, matrix diffusion is assumed to be Fickian and homogeneous, but
some authors suggested that these assumptions may not be valid [18]; the uncertainties in the migration
pathways for contaminants may make inappropriate the deterministic treatment of transport
and therefore stochastic methods have been also developed. Indeed, simple models do not include
the effect of the heterogeneity in rock properties. First attempts modelling the diffusion in a rock
matrix consisting of heterogeneous porosity patterns exist [19, 20, 21]. To validate these models, it
is necessary to obtain diffusion coefficients and to correlate diffusion profiles with the physical (and
mineralogical) properties of the rock matrix [22].
Thus, a microscale approach to matrix diffusion was started by combining diffusion studies and
characterization matrix porosity at a mineral scale [23]. The Rutherford backscattering spectrometry
(RBS) is a nuclear ion beam technique that allows measuring concentration profiles in a micrometric
scale with a resolution that allows measurements within a single mineral. Apparent diffusion coefficients
of uranium could be determined in three different granite types (Grimsel, El-Berrocal and
Los Ratones, both from Spain) in different minerals [23]. Differences observed are related to the
nature of the mineral grains, particularly grain porosity. The measured Da values for the U diffusion
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