RD&D-Programme 2004 - SKB

19 Geosphere
19.1 Initial state of the geosphere
The deep repository will be built in crystalline rock of granitic composition. The starting point
for the safety assessment is the situation that prevails when the repository has just been built and
closed. For the geosphere, this situation is a disturbance of the state that prevailed before the
repository was built.
Above all, drainage of the repository affects groundwater flow and pressure. The chemistry of
the groundwater can also be affected by the flow of groundwater, if water of a different composition
flows towards the repository. The chemical conditions are also influenced by the fact that
the repository is kept open and various materials are introduced. The magnitude of this influence
depends on several factors, for example how the repository is built and how long it is drained.
The results of the site investigation comprise the most important basis for determining the
post-closure state of the geosphere (the initial state). The programme for this has been presented
in special reports.
19.2 Processes in the geosphere
19.2.1 Overview of processes
Heat that is generated in the fuel is conducted out via the canister and the buffer and heats the
host rock. The groundwater is redistributed in the geosphere’s fracture system by groundwater
flow. Gas flow may also occur. A mechanical state exists initially in the geosphere which is
determined by the natural rock stresses and fracture systems on the repository site plus the
changes to which construction of the repository has given rise.
The mechanical evolution is determined by how the geosphere responds to the different
mechanical loads to which it is subjected. The loads may consist of the thermal expansion to
which the heating of the repository leads, the pressure from swelling buffer and backfill, effects
of earthquakes and the large-scale tectonic evolution. Changes in the geosphere may include
fracturing, reactivation (sudden movements in existing fractures) or rock creep (slow redistributions
in the rock). Movements in intact rock, i.e. compression or expansion of otherwise intact
rock blocks, also occur, along with erosion, i.e. weathering of the surface rock, particularly in
conjunction with glaciations.
The post-closure chemical evolution is determined by a number of transport processes and
reactions. The predominant transport process over long distances is advection, while diffusion
plays a great role over short distances and in rock blocks where the water is immobile.
In advection, solutes accompany the flowing water. The process leads to mixing of different
types of water from different parts of the geosphere. Reactions occur between the groundwater
and fracture surfaces, and these give rise to dissolution and precipitation of fracture-filling
minerals. Moreover, very slow reactions occur between the groundwater and the minerals in the
rock matrix. Microbial processes, degradation of inorganic materials from repository construction,
colloid formation and gas formation take place in the groundwater. During a glaciation,
methane ice formation and salt exclusion can also occur.
If radionuclides are released, they can be transported with the flowing groundwater by
advection. Diffusion can also be important if the water is immobile or moves very slowly.
An important aspect of this is matrix diffusion, i.e. radionuclides diffuse into the stagnant water
in the microfractures in the rock and are thereby retained and transported more slowly than the
flowing water. Sorption, where radionuclides adhere (sorb) to the surfaces of the fracture system
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