Euradwaste '08 - EU Bookshop - Europa

Figure 1: Schematic of the FEBEX drift with the new boreholes, the packed-intervals, the main
fractures and the locations of the Re and I tracers in the FEBEX experiment.
Most intervals of new boreholes, closer to the bentonite, showed higher concentration of the main
ions than the old radial ones. An increase of Na + and Cl - was observed in all the intervals of
FU05.001, particularly relevant in the packed-off section isolating a small lamprophyre dyke (interval
4, Figure 1). Based on the data obtained in these in-situ studies a mass-transfer conceptual
model was developed [9, 10]. Additionally, these studied allowed to determine the mean effective
diffusion coefficient for Cl - (De = 5.0E-11 m 2 /s). This result will be compared with Cl - diffusion
coefficients obtained in the large scale migration experiment that simulates the in-situ FEBEX configuration
(bentonite+ large block granite) (WP4.4) [11].
Distribution of the groundwater flow: characterisation and modelling
In crystalline rocks water flow takes place in the fractures which are the main conducting paths being
advection the dominant transport mechanism. The water flow in the porous rock matrix, with
low hydraulic conductivity, is negligible and the main transport mechanism here is diffusion. The
predominance of advection over diffusion depends on the characteristics of the fluid flow system.
Therefore, the characterisation of the fluid flow system is a key point for evaluating which paths are
actually available for RN transport and retention. Fracture network can be very complex and the
pore space can be connected or not. It is recognised that small scale features may have an important
influence on the overall transport behaviour so that a study of the rock pore space from m to the
dm scale was one of the objectives of WP 4.2.
At the in-situ scale (dm-m) the characterization of the granite in the FEBEX tunnel was carried out
with geophysical experiments, including Natural Gamma, Borehole Ground Penetrating Radar
(GPR) and Cross-hole Ultrasonic Tomography. The main objective of the work was to visualize the
geometry of the network of fractures in the region between the main boreholes in a quasi 3D-shape.
Different fractures cut both FU05.001 and FU05.002 boreholes, all showing low transmissivity
(1·10 -11 -1·10 -12 m 2 /s) with exception of the interval 1 of FU05.001 (6-8·10 -10 m 2 /s), at the back of
the gallery. The hydro-geological conceptual model of the FEBEX site (10s of meters) was updated
on a smaller scale (close to the granite-bentonite interface). Geophysical studies allowed identifying
three different fractured regions, slightly parallel to the gallery, and validating indirect visualization
methods for the determination of fracture network in a crystalline rock [12].
At a laboratory scale (WP4.2) several techniques were used for the characterisation of the pore
structure of different granite samples (Grimsel, Äspö, Olkiluoto, selected cores from the FEBEX
site). Different rock matrix characterisation methods (PMMA method, X-ray tomography, confocal
laser microscopy) were compared to highlight the applicability and limits of each technique [13].
330