RD&D-Programme 2004 - SKB

a so-called Markov-directed Random Walk technique. This technique, which is independent
of whether the distribution of (bv) –1 is power-law or not, will be further developed and tested
numerically for different discrete fracture networks. The purpose of this type of upscaling is
to be able to extrapolate in modelling contexts from smaller scales, where a discrete fracture
network model exists, to larger scales that are typically described with continuum models where
the transport resistance cannot be explicitly calculated.
19.2.13 Diffusion – groundwater chemistry
This section deals with the effects of molecular diffusion of groundwater components globally
in a very long time perspective and how the process affects the hydrochemical conditions.
Molecular diffusion and matrix diffusion and their importance for nuclide transport are dealt
with in the next section 19.2.14.
Conclusions in RD&D 2001 and its review
Site investigations in Finland have shown that the hydrogeochemical conditions in Olkiluoto
and in Äspö are similar. Meteoric water, old seawater and glacial water occur in varying proportions
in the rock down to a depth of approximately 500 metres. The salinity increases linearly
with the depth. At depths greater than 500 metres the salinity increase accelerates and the water
is estimated to have a residence time far in excess of 10,000 years. This indicates that a dynamic
process controlled by inflow of water from higher-lying areas and outflow in lower-lying
areas is going on down to a depth of 500 metres. At greater depths, the groundwater system is
unaffected by this dynamic. It is possible that the clear boundary between the dynamic and the
deeper water has been caused by diffusive transport that has occurred over a period of around a
million years. If the dynamic water has a measured residence time of 1,000 to 10,000 years, the
corresponding value for the stagnant water is much longer. The conclusion is that the deep brine
(saline water), which is mobile in a geological time perspective, can be regarded as stagnant in a
10,000–100,000 year perspective.
Newfound knowledge since RD&D 2001
The hypothesis has been forwarded that the deep brine was created by salt exclusion during
cold climatic periods /19-61/. The authors based their hypothesis on analyses of iodine-29 to
estimate the retention time for the deep brine at a sampling site in Canada at between 200,000
and 1.6 million years. Regardless of the origin of the brine, it tends to stay in place due to its
high density.
Programme
This field is not judged to require any extensive research, development or demonstration today.
Groundwater analyses and modelling studies aimed at determining what is required for the
deep brine to be permanent are planned, both in preparation for the next safety assessment and
within the site investigations. We will also investigate whether it is possible to utilize data from
Greenland, where there is a thick ice sheet, to check the models for groundwater flow and salt
distribution under such conditions, see also section 19.2.24.
19.2.14 Diffusion – radionuclide transport
Conclusions in RD&D 2001 and its review
The project for measurement of diffusivity in the field was presented in RD&D 2001, but results
were not available at that time. It was further stated that a consensus should be reached between
the concerned safety assessment parties in matters relating to retention, in particular matrix
diffusion.
258 RD&D-Programme 2004