RD&D-Programme 2004 - SKB

• Tests of the wetting process close to full water saturation. The time to water saturation
appears to be longer than expected in some cases, due to the fact that the processes in the
final phase of water saturation have not been carefully studied.
To verify the course of swelling, extrusion and homogenization during the interaction of the
buffer with the perforated container in KBS-3H, a nearly full-scale test will be performed in
the laboratory. A portion of a deposition hole with rock, perforated tube and bentonite will
be simulated. To reduce the dimensions but nevertheless maintain the full scale of the tested
components, only the bentonite portion will be tested, i.e. no canister will be included. this
means that a steel cylinder with an inside diameter of 0.8 m and a length of about 1.0 m will
be used to enclose and wet the bentonite, i.e. to simulate the rock. To facilitate dismantling, the
tube will be axially split. The deposition container (thickness and holes) and all gaps will be
made full scale. In order to monitor the wetting process, total pressure and relative humidity
(RH) will be measured at 6–10 points in the bentonite. Sensors will be placed on the outer tube
and in the centre of the bentonite. The volume of incoming water will also be measured. The test
is planned to start in 2004 and be terminated in 2006.
Continued follow-up and evaluation will be done of the large-scale Czech laboratory
experiment, the Mock-up Experiment. SKB is supporting both the test and its evaluation.
17.2.13 Advection
Solutes can be transported with pore water by pressure-induced flow – advection. The process
is of importance in the buffer during the unsaturated period when a net flow of water takes place
into the buffer. The most important requirement on the buffer material is that it should prevent
flow around the canister under saturated conditions. Solute transport in the pore water is then
dominated by diffusion, see section 17.2.14. Water flow in the buffer under unsaturated and
saturated conditions is dealt with in sections 17.2.4 and 17.2.5.
17.2.14 Diffusion
Solutes can be transported in stagnant pore water by diffusion, whereby substances move from
areas of higher concentration to areas of lower concentration. The process leads to redistribution
of solutes in the pore water and thus affects the pore water composition.
The diffusion process is strongly coupled to nearly all chemical processes in the buffer, since
it accounts for transport of reactants to and reaction products from the processes. Diffusion is
thereby a central process for the entire chemical evolution in the buffer. The process is included
in section 17.2.23 as regards the chemical evolution of the buffer and in section 17.2.25 as
regards radionuclide transport.
17.2.15 Osmosis
The physical properties of the buffer, including swelling pressure and hydraulic conductivity, are
intimately linked to the buffer’s ability to absorb and bind water. The binding force of bentonite
is mainly dependent on the proportion of montmorillonite and on variations in the mineral
structure of the montmorillonite. The binding force for a given bentonite material declines as the
quantity of absorbed water increases. The relationship can be measured and is usually described
with a so-called water saturation curve. Other components in a repository system – such as host
rock, salt in the groundwater and bacteria – can also bind water to varying degrees, leading to
competition for the water. The bentonite’s swelling pressure is thereby affected, which can be
described quantitatively with the aid of thermodynamics, e.g. in the form of osmotic pressure.
Conclusions in RD&D 2001 and its review
In SKI’s opinion, SKB has good insight into the processes of swelling and osmosis in the
bentonite clay.
216 RD&D-Programme 2004