RD&D-Programme 2004 - SKB

Figure 17-4. Natural redox front in the bentonite mine at Angeria on the Greek island of Milos.
17.2.18 Dissolution/precipitation of impurities
Besides montmorillonite, the buffer material also consists of accessory minerals, which are
counted here among the material’s impurities. In the repository environment, these impurities
can be dissolved and sometimes re-precipitated depending on the prevailing conditions. The
properties of the buffer can thereby be changed.
Most of the conceivable processes are well known in themselves and can be modelled for less
complex systems. However, conditions in the buffer with respect to transport and reaction
kinetics are not fully understood for all processes. Transport of substances can take place in
different forms and cannot be thoroughly described today. In particular there are remaining
modelling problems during the water saturation phase, when water is transported in both vapour
and liquid form. Modelling of the system must therefore be coupled to the THM processes that
proceed in parallel with transport and reaction kinetics.
Conclusions in RD&D 2001 and its review
Kasam underlines the need for further studies on the release, transport and precipitation of
dissolved silicon and other substances in the buffer.
Newfound knowledge since RD&D 2001
In the Lot tests at Äspö, buffer material is exposed to large temperature gradients (a maximum
of 130 to 80°C over a distance of 10 cm). Material from the three completed one-year tests
has been analyzed in a large number of points with respect to e.g. distribution of elements and
rheological properties. The results show that a minimal redistribution of silicon and magnesium
has taken place. The changes were so small that they cannot be related to specific minerals. The
bentonite was generally more plastic after the test, which shows that no cementation had taken
place in most of the buffer. On the other hand, a clear precipitation of gypsum in particular
220 RD&D-Programme 2004