RD&D-Programme 2004 - SKB

17.2.10 Mechanical interaction buffer/near-field rock
The mechanical interaction between buffer and near-field rock is caused by, among other things,
swelling pressure from the buffer, convergence of deposition holes and shear movements in the
rock. Convergence is dealt with in section 19.2.5 Movements in intact rock.
In KBS-3H the outer container will corrode. The transformation from iron to magnetite entails a
volume increase and an increased pressure against rock and canister.
Conclusions in RD&D 2001 and its review
Not dealt with.
Newfound knowledge since RD&D 2001
Effect of rock shear is dealt with in section 17.2.9.
Theoretical studies and model tests of the extrusion of the bentonite through the holes in the
container in KBS-3H have led to a theoretical model that can be used to optimize the hole
pattern and understand how the bentonite penetrates in behind the perforation. The hydraulic
conductivity of the bentonite between the perforation and the rock wall has been measured in
the model test (scale 1:10) to be about 10 –12 m/s, which is slightly higher than expected based
on the results of other measurements and of theoretical calculations, but still fully acceptable.
The model test is described further in section 17.2.12.
Scoping calculations for KBS-3H concerning the influence of extreme cases where the steel
container either doubles its volume or disappears completely due to corrosion show that the
buffer can be designed so that both extremes result in a density and a swelling pressure that lie
within the required values.
Programme
The large-scale laboratory test Big Bertha will simulate, for KBS-3H on a full scale, the
swelling-out of the bentonite through the perforated container and the interaction between the
bentonite and the simulated rock wall. Swelling pressure against the rock wall will be measured
in numerous points and, together with sampling after dismantling, provide additional information
on how buffer, container and rock wall interact.
17.2.11 Thermal expansion
When the temperature changes in the buffer, the volume of the pore water will change more
than the volume of the mineral phase. The pore water pressure rises when the temperature
increases, and temperature differences between different parts of the buffer thereby lead
to pressure differences, which in turn lead to movement of the pore water to equalize the
differences. In the interface against the backfill, the process can lead to upward expansion of
the buffer. The process is well understood for water-saturated bentonite. For non-water-saturated
bentonite the thermo-mechanical theory is not complete, but the consequences of this process
are deemed in this case to be unimportant for safety. Thermal expansion is included in the
coupled THM model, see section 17.2.12.
17.2.12 Integrated studies – THM evolution in unsaturated buffer
When the repository has been sealed, the buffer will absorb water from the surrounding
rock. The water uptake affects and is affected by a number of coupled thermal, hydraulic and
mechanical processes. Extensive experimentation and model development are being conducted
in this area within SKB’s programme.
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