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within feldspars in the three granites were comparable but the most interesting difference was found
between the various quartz grains. In some granite where quartz minerals showed no accessible
PMMA porosity, uranium access was subject to the existence of micro-cracks or intergranular fissures.
Figure 2 shows the RBS spectra of Grimsel granite obtained on quartz, feldspar and biotite.
In the Table below, the values of apparent diffusion coefficients obtained for each mineral type and
its porosity are shown. The RBS technique was also used to quantify Cs diffusion on single minerals
in Czech granites [24].
Energy (MeV)
Normalized Yield
7
6
5
4
3
2
1
1.4 1.6 1.8 2.0
Grimsel Quartz + U 1 day
Grimsel Feldspar + U 1 day
Grimsel biotite + U 1 day
Simulations
0
500 600 700
Channel
800 900
Sorption
“Sorption” is the general term used to define an unknown retention mechanism at a solid surface.
RN sorption may take place at the fracture walls, but also on the materials filling the fractures. In
PA, sorption is handled as a reversible attachment of dissolved species to surfaces using the “Kd
approach”. The Kd is experimentally derived, generally from static “batch” experiments under sitespecific
conditions. Pore surface of the rock matrix is considered to dominate sorption, while the
sorption on fracture or infills is minor and neglected in PA. Limitations of the Kd concept are fully
recognised: in particular, the Kd-approach do not take into account the chemistry of the pore solution
and its variability. Besides, other relevant processes as precipitation /co-precipitation and solid
solution formation may be hidden in Kd values. Mechanistic approach to sorption and retention
processes is widely treated in RTDC 1. In RTDC 4, two main problems related to Kd values are being
evaluated: Kd are not obtained directly on intact rocks [1] and, furthermore, the effect of the
heterogeneities is totally neglected.
Different “visualisation” techniques are available to observe the regions in which radionuclides interacts
(e.g. modern autoradiography method [24]), and to perform sorption studies on intact rocks.
The main challenge is to quantify RN retention at a mineral level, therefore sorption experiments
were carried out with small rock pieces using the particle induced X-ray emission technique
( PIXE). A mapping of the single elements on the solid surface allows identifying both the main
minerals present and the reactive areas where the RN is sorbed. The quantification of RN retention
in single minerals can be done only by specific analyses of the individual PIXE spectra in each
scanned point within 2x2 mm 2 areas and this methodology was developed in RTDC4. Small regions
within single minerals can be selected as shown in Figure 3 where uranium is preferentially sorbed
in a biotite. The variability of the surface distribution coefficient (Ka) was analysed as the studied
areas increased. This investigation tried to understand how the distribution coefficients must be upscaled
for consideration of the mineralogical heterogeneity found in any natural system.
332
Mineral Da (m2/s) Porosity
Feldspar (1.5 0.5) E-13 0.5 %
Quartz (1.1 0.5) E-13 0.5 %
Dark minerals (5.2 0.5) E-13 > 1.4 %
Figure 2: RBS spectra of Grimsel granite main
minerals after contact with U solution 1 day: (�)
Quartz, (�) Feldspar and (�) Biotite. Simulations
are plotted as continuous lines. In the table,
the values of apparent diffusion coefficients obtained
for each mineral type and its porosity are
shown.