RD&D-Programme 2004 - SKB

can be taken into account in a suitable manner in the type of thermal analyses that have now
been done /17-5/. The thermal analyses also need to be supplemented with analyses where
canister spacing is varied, for example due to the necessity to relocate canister positions to
avoid intersecting large fractures.
The problem of temperature offset across gaps and the need for temperature margins needs to be
further studied. In the first place, the ongoing tests (Prototype Repository) need to be evaluated,
and in the second place the reduction of the gap between canister and bentonite needed to
warrant a temperature margin of 10°C or less needs to be determined.
The calculation results currently available indicate that the data uncertainty margin does not
need to be 10°C. We need to revise and update the experimental data that underlie the assumed
dependency of the bentonite’s thermal conductivity on water saturation and porosity /17-8/.
The revision is needed in order to reduce the data uncertainty margins for the thermal design
of the repository and in order to improve the material models that will be used in further THM
modellings of the water saturation phase.
The problem with heat transport in the buffer will be dealt with in the THM calculations that are
planned within the framework of the different Äspö projects and thereby within the framework
of the activities of Äspö’s international EBS task force.
17.2.4 Water transport under unsaturated conditions
When the repository has been sealed, the buffer will absorb water from the surrounding rock.
The saturated buffer will build up a swelling pressure that mechanically affects the rock, the
canister and the backfill. Water transport in the unsaturated buffer is a complicated process that
is dependent on, among other things, the temperature and the smectite and water content of the
different parts of the buffer. The most important driving force for water saturation is a negative
capillary pressure in the buffer pores, which leads to water uptake from the rock.
The hydraulic conditions in the rock surrounding the deposition hole are thus crucial for the
course of the process. With an unlimited supply, full water saturation will be achieved between
canister and rock within a few years. A number of conditions in the rock are of importance for
the water supply.
Conclusions in RD&D 2001 and its review
SKI considers that unambiguous experimental data in support of SKB’s assumptions concerning
the saturation process are still lacking and that SKB should show how an adequate flow of water
to the buffer can be ensured (as well as the effects of high salinities). SKI also questions whether
SKB can draw any far-reaching conclusions on the natural resaturation of the bentonite from
only two short-term tests (five years) in the Äspö HRL.
Newfound knowledge since RD&D 2001
During the past few years, the water transport process has been studied in the laboratory, in the
field and in model calculations. Examples of such studies are given in the following.
The largest driving forces for water in unsaturated buffer are the negative pore pressure and the
temperature gradient. Factors that affect the negative pore pressure are therefore important for
water transport. The way in which the negative pore pressure varies with initial state, water ratio
and swelling pressure is being studied in a doctoral project, see further section 17.2.12. Water
transport in nearly saturated buffer (over 95 percent saturation) has also been studied in the
laboratory in conjunction with investigations concerning the wetting of the buffer in the planned
field test Lasgit. Furthermore, the wetting phase has been studied in scale tests of KBS-3H,
whereby a 1:10 scale model has been built of a tunnel section with two buffer-canister parcels
with spacer plugs.
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