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eal concentration gradient in samples. This observation is particularly important from a performance
assessment perspective since it suggests that the values of effective diffusion coefficients can
be site specific, i.e., different for groundwater with low and high ionic strength.
Under NF-PRO, various experiments were performed to assess the influence of iron corrosion products
on radionuclide mobility in the near-field. In particular, the question whether corrosion products
can reduce and immobilize elements such as Tc(VII), Se(VI), Se(IV), Np(V) was addressed.
For selenium, experimental data show that under anoxic conditions, Se(IV) and Se(VI) in groundwater
can be reduced by iron and/or iron corrosion products (FeCO3, Fe3O4 and green rust) to insoluble
Se(0) and Se(-I) (as FeSe2).
4.3 Advances in research related to Thermo-Hydro-Mechanical and coupled processes affecting
the near-field system
During the operational phase of a repository and for some time after closure, the near-field is subject
to strong thermal, hydraulic and mechanical gradients. These gradients will evolve towards
equilibrium. Processes controlling this evolution are strongly coupled and depend to some degree
on local chemical conditions.
In most repository concepts considered by EU Member States, buffer or backfill materials are emplaced
around the disposed waste canisters. In granite and clay host rocks, bentonite is applied as
buffer material while in rock salt, crushed salt is used. After emplacement of the bentonite buffer,
hydraulic gradients lead relatively rapidly to the hydration of the bentonite. Buffer and backfill materials
are associated with important safety functions since they limit transport of radionuclides that
may be released from the waste matrix and provide a favourable chemical and mechanical environment
with respect to other barriers (e.g. the waste form, the canister/overpack and the host rock).
An important part of the work performed by NF-PRO involved the investigation of the combined
effects of thermal, geomechanical and hydrological processes on the behaviour of bentonite buffer
and crushed salt backfill. A comprehensive analysis of the THM (C) processes in the EBS has been
carried out as under NF-PRO, taking into account various types of buffer material (bentonite, salt),
different scales (from laboratory to real scale), and several degrees of saturation and thermal states.
Under NF-PRO, a large number of experiments were performed to evaluate parameters and processes
that influence the hydration of bentonite buffer.
Progress in research related to the impact of THM processes on the bentonite buffer
In a first series of experiments, the evolution of the hydration of the bentonite buffer and the THM
evolution of the near-field during the transient period was investigated. Experimental work as part
of NF-PRO has provided a large body of qualitative and quantitative information on individual
processes relevant for the THM behaviour of the bentonite during the saturation-thermal phase.
This information includes the potential threshold gradient for water flow in bentonite, dependence
of permeability on temperature, water vapour movement. Data and information at parameter/process
level has been included in THM models in order to improve the predictive capabilities
for the transient THM phase. In a second series of experiments, tests were conducted in order to
assess the impact of the transient THM period on the long-term properties of the bentonite buffer.
These experiments have confirmed that the bentonite swelling capability remains basically unchanged
after several years of hydration with thermal gradients. Moreover, no mineralogical
changes were observed.
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