RD&D-Programme 2004 - SKB

pumped through the fracture for a long period. In the experiments, between 20 and 40 percent
of the injected neptunium accompanied the groundwater in the form of uncomplexed neptunyl
ion (NpO 2+ ) (same breakthrough as HTO). The longer the residence time in the fracture, the
lower the concentrations of pentavalent neptunium were observed in the eluate. No plutonium
was found in the eluate, while americium was found in concentrations on the order of 10 –11 mol/l
/19-83/. Similar experiments have also been conducted in above-ground laboratories. Within the
framework of RNR Actinide, batch sorption tests have also been performed in above-ground
laboratories with the same actinides as in the field tests and fracture-filling material/crushed
granite. These tests showed a clear time dependence for sorption of the actinides in question.
Some knowledge of sorption has also emerged from the True experiments. These results are
described below in section 19.2.26.
Knowledge of sorption and sorption mechanisms is being compiled in the ongoing EU project
Retrock /19-62/. The final report for Retrock will be completed during 2004.
Programme
Within the framework of actinide migration in a drill core with a longitudinal fracture, a test will
be conducted with uranium and technetium using the same experimental set-up as for actinide
migration (see above).
It has emerged from various joint international projects, such as Geotrap within the OECD/NEA
and Retrock within the EU, that a more process-oriented description of the retention processes
that are often lumped together under the heading of sorption may be needed for the purposes of
safety assessment as well. The processes that are principally of interest besides pure sorption
are precipitation, co-precipitation and surface precipitation. The processes that are normally
included in sorption are mainly ion exchange and surface complexation processes.
Process-based models can provide better process understanding and be used to bound simplified
models based on distribution coefficients at equilibrium (K d values). SKB plans to evaluate
the need for process-oriented sorption modelling specifically for safety assessment during the
current three-year period. If a need to develop this type of modelling capacity emerges, work
will be initiated. An obvious advantage of models of this type is that they can be used to study
how retention is affected by changes in groundwater chemistry, such as pH changes. A new
task has also been proposed for the Äspö Task Force on Modelling of Groundwater Flow and
Transport of Solutes that would involve reinterpreting the True-1 experiments with a more
process-oriented modelling strategy for retention. SKB is also continuing to support more
fundamental studies of surface complexation.
During the current period, a licenciate project will be conducted to develop methods for
determining K d values on intact pieces of rock with the aid of electrical methods. A problem
with traditional measurements of K d values is that they are done either on crushed rock material
or via diffusion tests on intact pieces of rock, which is more relevant but takes a very long time.
The idea of the planned project is to investigate whether an electrical potential can be used to
speed up the penetration of sorbing nuclides in intact pieces of rock. In preliminary tests with
non-sorbing substances, the penetration rate increased by around a factor of 1,000 /19-84,
19-85/.
In the site investigation programme, sorption measurements on site-specific materials are
also being carried out in the laboratory. These investigations are described in greater detail in
/19-73/.
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