RD&D-Programme 2004 - SKB

Newfound knowledge since RD&D 2001
The series of tests with gas transport in bentonite that has been under way since the mid-1990s
has been concluded and the results reported /17-9/. The questions to be answered were:
• At what gas pressure can hydrogen enter the bentonite?
• How much pore water is displaced by the gas?
• Is the gas dispersed or carried in discrete pathways?
• What controls the direction of the gas flux?
• What is the highest pressure that can be developed in the gas?
• Can the gas have any adverse effects on the buffer?
• How important are the boundary conditions for gas transport?
The most recent experiments have been carried out in a constant volume cell with gas injection
in the middle of the specimen, see Figure 17-1. In the walls of the cell there are three rows of
filters to capture the gas.
The tests showed that the gas pressure can rise to values that are much higher than the sum
of the swelling pressure and the water pressure. This is a departure from the tests performed
previously. It is therefore obvious that the boundary conditions are very important for the results
of gas tests in bentonite. The highest measured gas pressures were over 22 MPa, for a swelling
pressure of about 6 MPa and a water pressure of 1 MPa.
The experimental set-up with three sink filter arrays showed clearly that the gas is not dispersed
uniformly in the clay, but chooses to travel in discrete transport pathways. These pathways are
not stable, however, but can open and close for no apparent reason.
The tests have also shown that the gas is not capable of displacing water from bentonite;
60 litres of helium with a pressure of at least 8 MPa passed through a specimen with a diameter
of 60 mm and a length of 120 mm without any measurable water loss.
Nor is there anything to indicate that gas transport affects the hydraulic or self-healing
properties of the bentonite.
Vilton ‘O’ ring
Bentonite
Stainless steel filters
Remote porewater
pressure sensor
Cap screw
Push-rod
End closure
Injection
assembly
Axial stress
sensor
Annular tube
Pressure vessel
Figure 17-1. Equipment for gas tests.
Radial sink arrays
Radial stress sensor
206 RD&D-Programme 2004