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Q
B
Si
P
K
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333
Fe
Figure 3: Elemental distribution maps (Si, K, Fe and U) obtained by PIXE on a 2x2 mm 2 granite
area after the contact with uranium (10 days). Red squares refer to the areas selected to obtain the
individual PIXE spectra for a quantitative analysis, identified here as (Q) quartz; (B) biotite; (P)
plagioclase.
3. Transport in crystalline rocks: Effects of colloids.
To play a role in RN migration, colloids must exist in a non-negligible concentration, be mobile,
stable and be able to adsorb radionuclides in irreversible form [25]. These conditions must be verified
investigating scenarios, geochemistry, hydrogeology, and other physical factors as well as possible
artefacts that could bias data interpretation. In poorly mineralized waters, such as those present
at the GTS, bentonite colloids may fulfil several of the above-mentioned conditions, so that the
determination of the effects on radionuclide migration has to be studied in depth. The hydration and
loss of density of the bentonite backfill are necessary conditions for the colloids to be formed at the
bentonite/granite interface [4, 26] but the quantification, in realistic conditions, of the colloid source
term from the engineered barriers is still an open issue [1, 27].
In-situ transport studies at the GTS in the CRR project [5, 28] demonstrated that bentonite colloid
migration was not retarded with respect to the water flow; the colloid recovery depended on not
very well identified filtration processes taking place along the flow path as size exclusion,
rock/colloid interactions and diffusion in the rock matrix. The recovery of bentonite colloids and
highly sorbing tri- and tetravalent elements was very high [29] but water flow conditions were not
fully representative of those expected in a geological repository. Thus, it was considered necessary
to perform more experimental studies at a laboratory scale, under constrained conditions as similar
as possible to the real ones.
In RTDC4, different processes related to bentonite colloids were analysed: generation [30], stability
and effects on RN speciation (31), recovery under different flow rates [32], diffusion in the rock
matrix [33]. Other studies on transport and rock/colloid interactions related with colloid properties
(size or surface charge) were carried out also with model colloids (Au, quantum dots composed of
CdSe/ZnS).
The generation of bentonite colloids from compacted clay in contact with stagnant water was analysed.
Figure 3 (left) shows that colloid concentration initially increases but a steady state is
reached in a relatively short time. The higher the clay dry density, the higher the quantity of colloids
generated. Water chemistry controls the stability, size and concentration of the generated colloids:
the concentration of colloids increase, when the salinity of the water decreases (Figure 3,
right) and when pH increases.
+
U