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7. EDZ characterization and evolution in clay rock and salt
Studies within NF-PRO provide further important evidence of the occurrence of relatively rapid
self-sealing of the EDZ in clay host rocks. The hydro-mechanical self-sealing occurs irrespective of
the extent of the initial EDZ development. The kinetics depend, however, on the type and intrinsic
characteristics of the rock. For plastic clays, high water content, swelling capacity and plasticity of
clay minerals favour a quick and efficient self sealing, more rapid than in indurated clays.
Observations made on the Boom Clay seem to demonstrate that the EDZ self-sealing is accompanied
by a decrease of the overall porosity, a reduction of the connectivity of the micro-fracture network,
and consequently, a re-consolidation of the rock. Thus essentially complete self-sealing may
occur, with a total disappearance of the initial EDZ microfracture network.
For indurated clay rocks such as Opalinus and Callovo-Oxfordian Clays, the open fractures of the
EDZ are progressively closed, mainly as a result of hydro-mechanical effects, but they remain detectable.
This closing process has been shown to be sufficient in situ in Opalinus Clay and in Callovo-Oxfordian
core samples to restore a very low hydraulic permeability (close to that of the undisturbed
rock), but locally, the porosity may remain greater than that of the undisturbed rock.
Placed in the general context of the reference disposal systems, results confirm that the self-sealing
processes will start rapidly after the closure of the disposal cells, especially during the re-saturation
period. In addition to the hydro-mechanical loading, the self-sealing process will be helped by the
swelling of the clay-based (bentonite) buffer material (if included in the disposal concept), expected
before a significant chemical degradation of the metallic and /or cementitious material.
The impact of the long-term chemical interactions on the hydraulic properties of the rock and on the
solute and gas transfer through a possible remaining EDZ have not yet been fully assessed and integrated
in terms of disposal system performance. The alkaline perturbation from cementitious materials
may be significant and may extend several decimetres. It will likely contribute to a decrease in
the porosity (and permeability) of the “long term” EDZ, after the hydro-mechanical self-sealing
process (whilst possibly increasing the porosity of the concrete components). The specific characteristics
of this “new EDZ" are not precisely determined, thus the impacts on mass transport should
be further evaluated, in particular for the transport of gas from the repository system.
As noted in previous national PA studies, despite rather extreme representations of the EDZ in radionuclide
transport calculations, a continuous EDZ with a high permeability makes a negligible
radiological impact relative to the already low impact arising from transport directly through the
clay host rock. These findings have not eliminated the need for a sound scientific understanding of
EDZ-related phenomena. This is largely because PA models of the EDZ are simplified representations
of the relevant phenomena and there is a desire for a more convincing representation of these
processes, i.e. a more transparent basis for abstracting a PA model of transport through the EDZ
from a detailed scientific description of the processes. Clearly, due to the conservative assumptions
made in present PA studies, there is an expectation that more realistic representations of the EDZ in
radionuclide transport calculations will not significantly change the conclusions regarding the negligible
radiological consequences of an EDZ.
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