Euradwaste '08 - EU Bookshop - Europa

lower depths are less saline. For the purpose of the benchmark exercise a simplified hydrogeological
model for the overburden was developed that features some of the characteristics of the Gorleben
site without being an accurate model of the real situation there. In the model used two aquifers
are located above a salt dome and are separated by a clay layer. This aquiclude has one hydrological
window near the left side of the model region allowing groundwater exchange between the two aquifers.
At the bottom of the model region at one point radionuclides are introduced at a rate which
corresponds to the convergence-driven advective flow from a repository in rock salt which is been
completely filled with solution. This test case refers to a disturbed evolution of the repository system.
The evolution of the system over 1 million years and the transport behaviour of all pertinent radionuclides
in high-level waste have been calculated with the dedicated reactive transport program
r 3 t. In Fig.1 the results of model calculations are shown for the low-sorbing isotope Cl-36 for a
point in time of 10 000 years. The calculated concentrations are given in Becquerel per cubic metre
of water, colour coded on a logarithmic scale ranging from 10 -15 to 1. Note that the numbers given
on the colour bar give the according exponent. It can be clearly seen that there are two distinct preferential
flow paths for this radionuclide in the overburden. The first flow path, called here pathway
1, is from the radionuclide source in direct vertical direction through the lower aquifer, the aquiclude
and the upper aquifer. The second flow path, called pathway 2, is first in horizontal direction
to the left side in the lower aquifer towards the hydrological window in the clay aquiclude and than
through this window into the upper aquifer and to the surface. The two flow paths are indicated as
arrows in Fig. 1.
Fig.1 Cross section of the Cl-36-concentration, given in Bq/m 3 , after 10 000 years and the two
different abstracted 1D transport pathways of the CHET model
It was observed that the general transport behaviour of all radionuclides is similar, but that different
fractions are transported on the two pathways depending on their adsorption behaviour. While a
high fraction of a low-sorbing radionuclide like C-14 is directly transported upwards on pathway 1,
the highly sorbing radionuclides like Th-230 have only a very limited ability to be transported
across the clay layer. For these radionuclides pathway 2 was the predominating transport pathway.
In general, the r 3 t calculations showed that two distinct radionuclide concentration maxima exist at
the model surface, corresponding to pathway 1 and pathway 2, respectively. These two positions
reflect the locations with the maximum potential radiation exposures. The maxima only slightly
change their position with time.
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