Euradwaste '08 - EU Bookshop - Europa

nuclear waste within the near-field through experimental studies and both interpretative and predictive
modelling.
pH
14
13
12
11
10
9
8
7
High-pH external
solution added
6
100 200 300 400 500 600 700
Elapsed time (days)
184
External solution
5 mm
10 mm
20 mm
DIF-6
Figure 1: pH evolution of a diffusion cell experiment where pH of the external solution was shifted
to more alkaline conditions (from 8.3 to 11.7). Experimental data from [1].
2. Clay hydration, swelling and pore fluid evolution
Pore water characterisation is extremely difficult in samples of highly compacted bentonite, so that
typically pore water compositions are obtained by means of geochemical modelling. However,
there are potentially large uncertainties associated with some parameters, especially pH and redox
potential. In NF-PRO, a series of experiments was designed to enable measurements of these two
parameters in addition to full major element analysis, in highly compacted bentonite. These experiments
were aimed at the measurement of Eh and pH in bentonite pore water and their response
to changes in the composition of contacting external water [1]. The results indicate that shifting
redox conditions of the contacting external water to more oxidising conditions were only reflected
in the first 5 mm. When the conditions of this experiment were shifted to anaerobic conditions, the
Eh in the first 5 mm of the bentonite decreased to values equal to those of the rest of the bentonite.
These experimental results can be explained by reversible geochemical processes occurring in the
bentonite that partially buffer the redox of the system, likely enhanced by the high bentonite/pore
water ratio, impeding the penetration of the oxidising front further inside the bentonite. Similar behaviour
was identified when shifting the pH of external water to more alkaline conditions (pH =
11.7), where an increase in pH was only recorded in the first 5 mm of compacted bentonite. However,
in this case, after a sharp increase (Fig. 1), the pH in the first 5 mm of bentonite decreased
gradually until values between 9.5 and 10 were reached. This evolution can be explained by the
buffering effect induced by interaction with the bentonite minerals.
The effects of a thermal gradient on the hydration of unsaturated bentonite was examined in an experiment
consisting of a 60 x 7 cm column of unsaturated FEBEX bentonite at an initial dry density
of 1.64 g/cm 3 [2]. The bottom end of the column was heated to 100 ºC, while the top surface was in
contact with dilute, granitic-type water (I = 0.005 M) under an injection pressure of 1.2 MPa, to en-