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equilibrium conditions (associated with other problems), but there does not seem to be any
clear reason why they should not have a similar behaviour. The work on pure clay systems
provides a sound basis for partitioning the mass of both anionic and cationic RN between
different porosity volumes (anionic exclusion, interlayer, EDL, bulk).
Do we have a coherent conceptual model describing diffusion-driven transport of anionic
and cationic RN in clayrocks?
Here the answer is clearly mixed. For anions, yes. The results of the studies carried out at
scales ranging from molecular/microscopic, to mesoscopic, to macroscopic, and geological
formation scales offer a sound scientific basis for explaining and modelling migration of
anionic RN. As for cations, the picture is not so clear, with all of the results tending to
show that coupled diffusion-sorption migration is much more complex than expected, leading
generally to greater mobility than that predicted by coupling Fick and batch Kd. Several
hypotheses have been advanced for this, perhaps the most plausible being that cationic RN
diffuse along more than one type of ‘pathway’ (or porosity) in a clayrock, each having a
corresponding Dp value and sorption site population. In this case, mass transport kinetics
could limit access to the sites in the lower Dp porosity. It should also not be forgotten that
these studies are necessarily carried out on very small rock volumes, with the accompanying
possibility that effects of mineral-porosity heterogeneity existing at this scale might
also have an influence. It is not impossible that the reduced effect of sorption retardation
observed at these mm scales becomes less important when migration over larger space (and
time) scales are considered. In any case, more research is indicated in this area.
Do we have credible strategies / methods for carrying out the up-scaling needed to obtain
representative parameter values usable for performance assessment simulations of a
clayrock geological barrier system, in particular taking into account the effects of spatial
heterogeneity of rock physical-chemical properties.
Here the answer is an unqualified yes, backed up by the multiple lines of argument and
demonstration provided by the theoretical, experimental, up-scaling and natural tracer studies
presented above.
6 Acknowledgements
This project has been co-funded by the European Commission and performed as part of the sixth
Euratom Framework Programme for nuclear research and training activities (2002-2006) under contract
FI6W-CT-2004-516514.
References
[1] ONDRAF/NIRAS (2001), Safety assessment and feasibility Interim Report 2 (Safir 2), Nirond
2001-05
[2] Nagra (2002), Project Opalinus Clay, Demonstration of disposal feasibility for spent fuel,
vitrified high-level waste and long-lived intermediate-level waste, Safety Report, Technical
report 02-05
[3] Andra (2005), Dossier 2005 Argile
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