RD&D-Programme 2004 - SKB

These processes have been studied in the field mainly in conjunction with the wetting of
the buffer in a number of full-scale tests in the Äspö HRL (Prototype Repository, Canister
Retrieval Test and TBT). The water inflow and the change in the negative pore pressure can be
determined by measurement and follow-up of the relative humidity in a large number of points.
The temperature gradient is large in TBT, enabling the other major driving force for water,
which leads to drying-out in the hot part, to be studied.
An important update of computation capacity has been achieved by the purchase and testing of
the finite element code Code Bright developed at UPC in Barcelona. The code is an excellent
complement to Abaqus, since it models certain processes in water-unsaturated soil better. For
example, it models all three components (particles, water and gas) in unsaturated soil, whereas
Abaqus does not model the gas component (except for temperature-driven vapour flow). On the
other hand, due to the well-developed models in Code Bright, the material models contain more
parameters which are often difficult to determine. A great deal of work remains to be done in
determining and validating these models for SKB’s buffer materials.
Model calculations of water flow have mainly been done in conjunction with modelling of the
field and laboratory tests. The following calculations in particular can be mentioned:
• Modelling of TBT with existing models with both Abaqus and Code Bright and comparisons
with measurement data. Evaluation is under way.
• Modelling of the scale test of KBS-3H (1:10) with Abaqus. The final phase of the wetting
went slower than predicted.
• Updated calculations of the Canister Retrieval Test. Evaluation is under way.
• Modelling of the saturation and maturity phase for Lasgit with Abaqus. A new and simplified
material model developed for nearly saturated soil (over 95 percent) is used in these
calculations. The model has been partially verified by comparisons between laboratory tests
and modellings of the laboratory tests.
• Modelling of the first 1,000 days of the Febex experiment in Grimsel, Switzerland. The wetting
of the buffer and the thermo-hydro-mechanical effects in the rock have been modelled
and the results compared with measurements. The most important result from a water transport
viewpoint is that the wetting of the buffer agreed well with the measurement results,
even for the simplified assumption that the rock acts as a filter with zero water pressure
and supplies the bentonite with the water it needs. Another observation is that the measured
hydraulic interaction between rock and buffer is not as strong as has been modelled due
to the fact that the modelled media have been coupled together. A skin zone is needed to
explain the absence of negative water pressures in the rock.
Programme
The wetting process is a part of the THM modelling that will be included in the upcoming
research programme. The programme is described in section 17.2.12 “Integrated studies – THM
evolution in unsaturated buffer”.
17.2.5 Water transport under saturated conditions
Water transport in a saturated buffer is a complex interplay between a number of sub-processes
on a microscopic scale. On a macroscopic level, the result is that the permeability of a saturated
buffer is very low, and this is also the essential result for the safety assessment. Other uncertainties
concern the effect of transformations, which are expected to result in higher hydraulic
conductivity. Very high salinities are also of importance for the process, see section 17.2.15.
A special case of water-saturated buffer may arise if the water pressure in the buffer is equal
to the total external pressure. The effective pressure between the clay particles is then zero
and the buffer loses strength and enters a state of liquefaction. In the case of certain soils (soft
sand and soft illitic clay), this can occur as a result of strong vibration or water flows, but great
204 RD&D-Programme 2004