Euradwaste '08 - EU Bookshop - Europa

4.2 Advances in research related to the chemical evolution of the engineered barrier system
The Engineered Barrier System (EBS) isolates the disposed radioactive waste from the geosphere.
Diverse processes including the uptake of groundwater, chemical interactions between different
near-field components and the formation of secondary phases and corrosion products, influence the
confinement properties of the EBS and control radionuclide transport in the near-field. NF-PRO has
investigated a range of processes affecting the chemical evolution of the EBS, in particular the evolution
of the composition of porewater in the bentonite barrier, concrete-bentonite interactions, the
corrosion of metal-based repository components including the interaction of corrosion products
with bentonite, and radionuclide mobility in the bentonite barrier.
Progress in research related to processes affecting the bentonite porewater composition
Clay-based (bentonite) buffer materials are widely applied in repository concepts in granite and
clay host rocks. The bentonite buffer will progressively hydrate and act as a low permeability barrier
limiting and controlling radionuclide transport. An important part of the work by NF-PRO has
been dedicated to the investigation of the porewater chemistry and processes and mechanisms having
an impact on the evolution of the bentonite porewater composition. Changes in porewater composition
are particularly relevant to evaluate - amongst others - the dissolution of the waste matrix,
radionuclide mobility and canister corrosion rates. These compositional changes can be initiated by
internal factors, for example by a thermal gradient or by chemical interactions between different
repository components, or by external factors, for example changes in the composition of groundwater
entering the near-field system from the host rock following alteration of the host rock. NF-
PRO has investigated the mechanisms affecting porewater chemistry and radionuclide mobility to
evaluate the time-dependent evolution and the overall performance of the near-field system.
Eh and pH are key parameters with respect to chemical conditions in the near-field: Eh and pH have
major effects on concentrations of aqueous species (for example HS - , HCO3 - and Cl - ), which may
affect corrosion of metal-based engineered barriers and/or enhance radionuclide solubility. Until
recently, obtaining reliable data on redox conditions in compacted bentonite was very difficult since
different laboratory techniques tended to disturb the system and introduce sampling artefacts into
the measured data. Major advances have been made by NF-PRO regarding the direct measurement
of Eh and pH in compacted bentonite. Electrodes capable of producing accurate measurements of
Eh and pH in compacted bentonite were developed and used to assess the response of the bentonite
buffer to changes in pH (pH 11.7) and redox potential (aerobic) of contacting external groundwater.
Using cells with a total length of 20 mm, oxidising and alkaline perturbations were measured only
in the first 5 mm of the cells. pH-values re-established to values around 9 (the initial pH value was
equal to ~8.5) in the first cm of the bentonite and the extent of redox pertubations is limited. These
experimental observations indicate that redox and pH conditions are buffered very effectively in
compacted bentonite.
NF-PRO has investigated the interaction between saline porewater and the bentonite buffer. In particular,
the amount of water absorbed in different types of bentonite (MX-80 and FEBEX bentonite)
was measured as a function of porewater salinity. It was confirmed that the distribution of the water
among external and internal (interlamellar) sites depends on the pore water salinity. Results from
experimental work indicate that the amount of internal water decreases with increasing salinity.
Additional experiments have led to the conclusion that the amount of water absorbed in bentonite
depends on the dominant cation in the smectite exchanger. Experimental results with homoionised
FEBEX bentonite show that maximum water absorption occurs with magnesium or calcium as the
dominant exchangeable cations. Minimum water absorption was observed with potassium as domi-
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