RD&D-Programme 2004 - SKB

Swelling properties
The buffer must be able to swell to fill the space between canister and rock and to seal openings
that may be caused by thermal and tectonic effects. The requisite expansion capacity of the
buffer is estimated to correspond to a swelling pressure of at least approximately 1 MPa,
which presumes a density of at least 1,900 kg/m 3 for MX-80 in the water-saturated state.
Long-term stability
Commercial bentonites are natural materials that were often formed tens of millions of years
ago. This does not automatically mean that bentonite is stable in the repository environment.
The investigations of the long-term properties of bentonite that have been done within and
outside of SKB’s programme show, however, that compacted bentonite can retain its favourable
properties for long periods of time and under varying chemical and thermal conditions.
Microbial properties
It has been found that bacterial growth can occur in MX-80 buffer with a density of up to
1,700 kg/m 3 at water saturation, while 1,900 kg/m 3 does not allow any possibility of survival or
reproduction of bacteria of the kind investigated in SKB’s research. This means that the latter
density can be regarded as the minimum suitable.
Gas conductivity
The experiments that have been conducted under SKB’s auspices indicate that MX-80 bentonite
can open up and release large quantities of hydrogen gas, which may be formed by corrosion
of the iron insert in a defective canister. Unacceptable pressures in the canister and against the
buffer can thereby be avoided in such a situation.
Deformation properties
The most important deformations in the buffer are upward expansion by displacement of the
tunnel backfill and shear as a result of displacements in the rock. The upward-directed expansion
can lift the tunnel floor, with fracture widening and greatly increased hydraulic conductivity
as a consequence.
Displacements in the rock can take place in the form of tectonic or thermally induced shear of
fractures that intersect the deposition holes. Practical tests with MX-80 clay with a density of
up to approximately 2,050 kg/m 3 and application of a semi-empirical rheological model have
shown that anticipated rock movements do not cause buffer deformations that give rise to
canister damage.
Thermal properties
The buffer’s capacity to transfer heat from canisters to rock is mainly important in that too low
a thermal conductivity gives rise to high buffer temperature. This leads to altered solubilities
of the buffer minerals and a vapour pressure that can cause expulsion of water vapour from the
buffer through the overlying tunnel backfill. To minimize the negative effects of an excessively
high temperature and temperature gradient, the maximum canister temperature has been set at
100°C.
Fabrication of buffer blocks is described in Chapter 10.2.2. Fabrication aspects are discussed in
the following only insofar as they have a bearing on the research programme presented.
Conclusions in RD&D 2001 and its review
In its review of RD&D 2001, SKI points out that the buffer is essential for the repository,
especially as protection for the canister, that SKB’s programme for the buffer appears to be
194 RD&D-Programme 2004