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that in the experiments the bentonite columns were far from full saturation. Experimental data
available on this topic should be identified and new experiments proposed to clarify the expected
range of density variations in the bentonite buffer after saturation.
It is noted that a number of THM studies involving hydration and heating (e.g. FEBEX) were in
progress long before the NF-PRO project began. The comparison of models with experimental results
from these studies has proven valuable. In some cases, reasonable agreement has been obtained,
but in other cases there appear to be significant difficulties in obtaining agreement between
experiments and models. These differences should be explored further. Nonetheless, from the assessment
perspective it is not clear how significant these problems are when one considers the
somewhat tenuous relationship between the key safety functions of buffer and the processes being
modelled in these laboratory and field studies. It is worth reinforcing that safety functions of a bentonite
buffer are related principally to permeability, swelling capacity, and sorption, which are
properties that can be quantified. If these are within a suitable range, then the safety function can be
achieved. The THM models in general do not adequately characterize these parameters – post-test
analysis is clearly also needed. Thus one must judge adequacy of barrier performance using a number
of approaches and the predictive capability of THM models is only one aspect to be considered.
For salt repositories, porosity evolution around canisters and in seals has been an important issue in
PA. Results in NF-PRO show that at relatively low pressure (10 - 20 MPa), the water content in
granular salt is high enough to allow effective compaction to occur. Experiments indicate that pressure
solution may be the dominant deformation mechanism, though the mechanistic basis and role
of recrystallization are not yet fully understood.
Investigations performed in NF-PRO have led to a better understanding of the mechanical and hydraulic
behaviour of salt backfill materials especially in the range of low permeability. Different
constitutive laws have been tested and it was found that the Zhang model is appropriate for describing
salt bricks. Use of the Spiers model requires some more experimental work.
It was found that highly compacted crushed salt differs in its hydraulic properties from naturally
grown rock salt, even at comparable porosities. While rock salt is practically impermeable, the
compacted samples always show a measurable permeability. It seems to be the case that a network
of connected pores remains present even in highly compacted salt grit, while it does not exist in
natural rock salt.
The compaction behaviour of salt highly depends on the moisture content of the backfill material.
This is a well-known fact, but the investigations have contributed to enhancing the understanding of
this phenomenon and quantifying it, especially at low porosities between 1% and 10%.
Bentonite as an additive to crushed salt backfill is a means to enhance the compactibility since it
acts as some kind of lubricant between the salt grains. Additionally, the permeability of the material
mixture is reduced in comparison with pure crushed salt at the same state of porosity. Such a mixture
can be used in low-temperature regions of the repository.
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