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Regarding diffusion studies performed in NF-PRO, the most important question with direct implications
for PA is the apparent difference in effective diffusion coefficients (De) for neutral HTO and
positively charged species, suggesting that positively charged species can move in compacted bentonite
faster than neutral HTO. This effect was identified a long time ago, and has been ascribed by
some to a transport mechanism for cations in clay materials called “surface diffusion”. In similar
experiments, some groups do measure De values for cations much higher than for HTO while others
do not. Although the differences are not large in the context of the uncertainties that they introduce
into transport and dose calculations, they indicate a lack of understanding that warrants some
further study.
Modelling of the interactions of concrete with bentonite has been further studied within NF-PRO
using pure kinetic approaches and equilibrium-based approaches. Depending on the approach, the
mineral conversion is either slight and may extend over a few metres distance or substantial and be
limited to a decimetre range. The calculated effects and extent of the alkaline perturbation within
bentonite are highly sensitive to the feedback of chemistry on transport (i.e. clogging), to the model
for mineral buffering (e.g. montmorillonite dissolution), to the diffusion rate, as well as to the position
and dimension of the alkaline reservoir (renewal of aggressive solutes by diffusion). Further
studies need to be performed to reduce the uncertainties, notably regarding kinetic and equilibrium
thermodynamic data. Overall, it would be worth considering more realistic assessment of the effect
of chemical interactions between near-field components in systems with heterogeneities, e.g. in
terms of component interfaces, including impacts of the thermal transient.
Some studies regarding the effect of an alkaline plume on the safety-relevant properties (sorption,
porosity and diffusion coefficients) of the bentonite or clay rock would be useful to put into perspective
the relevance of the alkaline plume for repository safety. If these effects are found to be
negligible (similar Kd values and fluxes) or even beneficial (clogging of porosity that may reduce
mass transport) the efforts to be spent studying the alkaline plume should be smaller than if there
appear to be strong detrimental effects on safety-relevant properties bentonite and host rock.
6. THM processes in the EBS
A number of laboratory and URL experiments at various scales involving hydration and heating of
bentonite have been performed over a period of many years and these were modelled within NF-
PRO.
The “standard THM model” was able to make good predictions for THM behaviour of bentonite in
FEBEX “mock-up” and “in-situ” experiments. Trends are well predicted but the calculated longterm
hydration rates are greater than the measured values, the main disagreement being that after 3
years, hydration of the hotter areas of the bentonite proceeds more slowly than predicted by the
model. Further model testing should be performed, as the ability of the “standard THM model” to
explain the long-term experiments results seem not to have been fully exploited. The effect of parameter
uncertainty on predictions should be thoroughly analysed, in order to explore the full capabilities
of the “standard THM model” to predict results. Furthermore, when developing the model
and adding a new process or using a different set of parameter values, the whole experimental database
should be considered, rather than a single experiment. This would allow drawing general conclusions
regarding the occurrence of any new postulated processes.
Another uncertainty relates to the degree of heterogeneity in density that may remain in bentonite in
the long term. This uncertainty regarding the final state of the barrier was already identified at the
start of NF-PRO, and NF-PRO results confirm this uncertainty, but it must be taken into account
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