RD&D-Programme 2004 - SKB

Conclusions in RD&D 2001 and its review
Kasam thinks it is gratifying that SKB is conducting research on wetting and temperature
differences in the buffer with subsequent heat transport as a function of time and water quality.
SSI comments that SKB should give an account of how initial defects and the short-term
evolution of the buffer in the resaturation phase can influence the long-term performance of
the repository and to what extent the experiments at the Äspö HRL might be expected to yield
answers to these questions.
SKI finds that an urgent issue in this context is the resaturation phase for both the buffer and
the backfill, for which the experiments at the Äspö HRL play an important role. SKB needs to
clarify the expectations on the experiments and the evaluation criteria that will be used.
SKI considers it doubtful whether there will be time to finish the remaining work by the
scheduled time for repository licensing and points out that validated models for combined
heat and water transport in unsaturated buffer remain to be developed.
Newfound knowledge since RD&D 2001
Model studies
An integrated model for simulation of unsaturated coupled THM processes has previously been
developed and applied to the finite element code Abaqus. This model has proved less sufficient
for certain applications. Formation, condensation and transport of water vapour cannot be
modelled in a physically correct manner, nor can the pressure effects of generated or trapped
gas. Both water vapour and other gases are of importance for heat transport and water saturation.
For simple and idealized conditions, for example if the possibility that water vapour may leave
the buffer to condense in the backfill is not taken into consideration, it was possible to simulate
the effects of temperature-driven vapour transport fairly well with the calculation models
previously used without explicitly modelling the vapour phase. Effects of other gases could
not, however, be taken into account.
Inventory, evaluation and use of alternative calculation models has therefore been done. The
codes that have been tested are Compass, which was developed at the University of Wales in
Cardiff, and Code Bright, which was developed at UPC, Universidad Politécnica de Cataluñya,
in Barcelona. In these codes, the effect of negative pore pressure on the stress state (and thereby
the hydromechanical processes) can furthermore be handled according to modern models for
unsaturated soil materials.
As a result of the evaluation, Code Bright has been chosen and adapted to SKB’s buffer
material. Code Bright has been developed specially for analysis of THM processes in porous,
unsaturated materials. A study of the water saturation process in KBS-3V has been carried out
and the results reported /17-13/. The results showed that the area between canister and rock
wall becomes saturated with water in two to four years, provided there is a supply of water
at atmospheric pressure in the buffer-rock transition. To be able to fully take into account all
aspects of the water saturation process, however, it is necessary to consider not only those
aspects that have to do with T-H couplings, gas movement and vapour diffusion, but also the
hydromechanical aspects.
The new model has been applied to the TBT test at the Äspö HRL. The TBT test is being
conducted by Andra with SKB as the organizing and executive partner and concerns THM
processes at high temperatures (over 130°C). The new model produces physically realistic
results, including considerable vaporization and drying-out at the hot canister edge, vapour
diffusion in the direction of the thermal gradient and condensation in colder parts of the buffer.
The measurement results that are gradually emerging confirm the relevance of the model /17-14,
17-15/. The effect of the dry air that is present from the start in the pore system and in joints and
gaps is not quite clear, since it has not been possible to rule out the possibility that some of the
dry air might have leaked out from the experiment.
212 RD&D-Programme 2004