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U. Alonso
1
, T. Missana
1
, M. García-Gutiérrez
1
, A. Patelli
2
, J. Ravagnan
2
, V. Rigato
2
1
CIEMAT, Avda. Complutense 22, Edif. 19, 28040 Madrid, SPAIN
2
LNL - INFN, Via dell’ Università 2, 35020 Legnaro-Padova, ITALY
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Rock matrix diffusion is one of the possible mechanisms for radionuclide retardation in a
deep geological high-level radioactive waste repository, and it is usually considered that
radionuclides diffuse as solutes through the rock. Nonetheless, the potential effects that clay,
from the bentonite barrier, may induce on the radionuclides migration should be taken into
account. Furthermore, transport models generally assume that the whole mineral surface is
accessible to transport, whereas transport is highly conditioned by the heterogeneous mineral
distribution, since different minerals may act as preferential pathways, while others may present
higher sorption capability. It is therefore necessary to determine the actual surface area
accessible to transport.
The aim of the present work is the identification of the uranium preferential pathways to the
granite, both in presence or absence of bentonite clay. Results showed that uranium as solute
diffused in specific mineral areas, indicating that the actual surface area accessible to matrix
diffusion, and /or sorption on the surface, is significantly lower than the whole mineral surface.
By the other hand, the uranium in presence of the clay was randomly distributed on the surface,
and penetrated into the granite mainly through "defects" (as fractures or grain boundaries); its
migration being enhanced on specially fractured or disturbed areas.
,1752'8&7,21
One of the possible options considered in Spain for the geological disposal of radioactive
waste is a repository in a granitic rock. In this concept, a layer of compacted bentonite would
surround the canisters containing the waste [1]. The radionuclides transport in the rock is
expected to take place by advection and dispersion in the fracture network. Rock matrix
diffusion and sorption onto the mineral surfaces are the solute retardation mechanisms usually
considered and many diffusion studies, in fractured flow systems, state the importance of
accounting sorption and matrix diffusion [2]. A full understanding of the diffusion and/or
sorption processes in crystalline rocks requires the study of the physical and chemical properties
of the heterogeneous matrix surface, and fractures therein. Different minerals could act as
preferential pathways, due to the heterogeneous charge distribution of the rock surfaces.
However, despite models considering theoretically this heterogeneity exist [3-4], almost no
experimental studies concerning heterogeneous diffusion are available [5]. Furthermore, at the
bentonite /granite interface of the repository, the clay might play a role on the transport, due to
its intrusion in microfractures [6] or to the detachment of clay particles and clay colloid
formation [7].
The aim of the present work is to study the differences in the heterogeneous diffusion
behaviour of uranium when it is present as a solute or adsorbed onto bentonite and, in particular,
to analyse the extent of uranium diffusion and to evaluate the preferential diffusion paths. This
will allow elucidating the actual surface area accessible to matrix diffusion for both cases.
Mat. Res. Soc. Symp. Proc. Vol. 807 © 2004 Materials Research Society
1