RD&D-Programme 2004 - SKB

Q3
Tunnel
Crushed rock - bentonite
Canister
Bentonite
Q2
Disturbed zone
Damage (hole)
Rock
Deposition hole
Fuel
Q1
Bentonite
Channel
Canister wall
Fracture zone
Q4
Figure 14-4. Schematic view of the near-field, with the transport pathways used in SR 97.
Figure 14-4 shows how the deposition holes and the backfill in the deposition tunnel were
modelled in SR 97. In the model, four different flow paths, Q1–Q4, were used for the interface
between near-field and far-field.
To represent the barrier system through which radionuclides are transported, Comp23 makes use
of an integrated finite difference method and the compartment concept.
The far-field model
The far field model for SR-Can is similar to the one used in SR 97. This model, Farf31, /14-8/
calculates the transport of dissolved radionuclides through fractured rock, the retention caused
by interactions between the nuclides and the rock matrix, and radioactive chain decay.
The biosphere model
Radionuclide transport through the biosphere and doses to man were handled in SR 97 by the
Proper sub-module Bio42. This module was based on nuclide- and ecosystem-specific dose
conversion factors (EDFs), which convert nuclide releases to an ecosystem into doses. In SR 97,
the EDFs for the six ecosystems considered (well, lake, river, peat, soil, coast) were calculated
in a separate code /14-10/ using generic data for the ecosystem parameters. The different
ecosystems were represented by compartment models where each compartment corresponded
to a distinct part of an ecosystem. Figure 14-5 shows the example of a peat bog modelled as
two compartments, representing nuclides dissolved in water and nuclides sorbed to organic
and/or solid material, respectively. The nuclide exchange between the two compartments was
modelled using ecosystem- and element-specific distribution coefficients (Kd). In the modelled
peat bog, Figure 14-5, both nuclide inflow and outflow take place in the soluble phase and are
therefore controlled by the water balance. Depending on the water exchange at the studied site,
in- and outflow may take place to other ecosystem models, inflow may take place from release
points from the geosphere’s streamtubes, or outflow of nuclides may take place from the model
domain. However, in SR 97, only single ecosystems were considered with inflow only from
release points from streamtubes.
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