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Several experiments were performed to obtain information on the effect of canister corrosion products
on radionuclide sorption. Experiments were conducted to study sorption of Cs onto magnetite,
assuming that this is the main corrosion product in anaerobic conditions. The results showed that
sorption of Cs was negligible up to pH 8.5. Maximum sorption values were reached around a pH of
12. A significant increase in Cs sorption was observed with decreasing the ionic strength. Linear
sorption isotherms were observed for a Cs concentration within the range used (i.e. up to 10 -6 M).
As expected, Cs showed very minor sorption on the oxide. All experimental results could be satisfactorily
described with a simple model. The model developed was also capable of reproducing
sorption of Cs on magnetite in more complex solutions (bentonite and cement synthetic pore water).
Advances in studies related to concrete-bentonite interactions
Experiments investigating the evolution of pore water composition due to the interaction with concrete
have provided further evidence for the buffer capacity of bentonite. Experimental results have
shown that using an external solution with pH 13.5, the bentonite pore water reaches a maximum
pH of 9 after experimental durations of 596 days, indicating that pH is buffered by geochemical
processes in the bentonite. Further experiments were performed to obtain information on mineralogical
changes due to interactions between bentonite and high pH groundwater. At high pH´s (pH
� 13.5) minor mineralogical changes were found to occur in the bentonite. These changes are limited
to an alteration zone of a few millimeters from the interface after two years of experiments. The
most important changes are related to partial dissolution of montmorillonite (at expected rates of 10 -
12 to 10 -13 mol m -2 s -1 ), formation of saponite-type clay and the precipitation of zeolites (in some of
the experiments) and brucite. The most relevant change in all these experiments conducted under
repository conditions (diffusion experiments) consisted in a reduction of the CEC from 25 to 50%
of the initial value close to the interface with the concrete. Modelling of the interaction between
bentonite and concrete has been successful and was capable of reproducing the main processes,
such as partial pore blocking, brucite precipitation, minor montmorillonite dissolution, and the replacement
of Mg- by K-montmorillonite.
Advances in research concerning radionuclide transport in bentonite
NF-PRO's programme of work included a range of experiments investigating radionuclide transport
in bentonite materials. These experiments were performed to develop a mechanistic understanding
of the sorption and the diffusion of anions and cations in compacted bentonite. The experimental
programme allowed assessing the effect of the degree of compaction and porewater chemistry,
combined with the microstructure of bentonite.
Experiments were performed to determine the sorption of Ni(II), Eu(III) and U(VI) onto Namontmorillonite
as a function of carbonate concentration and pH. Data obtained from these experiments
indicate that Ni(II) sorption onto montmorillonite is rather insensitive to the presence of inorganic
carbon whereas the presence of inorganic carbon decreases sorption of Eu(III) and U(VI)
on montmorillonite. Experiments using Volclay bentonite at pH 8 show that Th sorption on bentonite
decreases in the presence of organic matter.
Radionuclide diffusion processes in bentonite were investigated on the basis of a series of throughdiffusion
and out-diffusion studies with 36 Cl - , 22 Na + and 85 Sr 2+ and 134 Cs + using Volclay KWK bentonite.
Experimental data indicate that values of effective diffusion coefficients for both cations and
anions depend on ionic strength of water in pore spaces. In the case of anions, an increased ionic
strength leads to an increased transport rate whereas in the case of cations, a decrease of transport
rate was observed. This is explained by difference between concentration gradient in reservoirs and
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