RD&D-Programme 2004 - SKB

Newfound knowledge since RD&D 2001
The conditions in a deposition hole with bentonite and groundwater are typical for systems
that can be described with Donnan equilibrium, which is characterized by the fact that an ion
cannot diffuse freely in the system, usually due to its size. In highly compacted bentonite, the
charged individual montmorillonite layers have very limited mobility due to their size and the
large quantity of layers. The prerequisite for equilibrium in such a system is that the product of
the activities of the diffusible ions is equal in the groundwater and in the pore water between the
bentonite layers, and that electrical neutrality prevails in both compartments. The natural and
relatively high negative charge of the mineral layers is compensated by counterions. In compacted
bentonite, the distribution between bentonite and water entails a counterion concentration
of several moles per litre. The ion product between the mineral layers will therefore be dominated
by the montmorillonite’s counterions. An increase in the groundwater’s ion concentration
therefore leads to a much smaller increase in the ion concentration between the mineral layers.
The difference in concentration increase gives rise to a new osmotic equilibrium, which leads to
a reduction in the bentonite’s swelling pressure. The pressure changes can be calculated, since
the activity of counterions and ions in the groundwater can be determined.
Extensive laboratory experiments have been conducted in cooperation with Posiva, Finland. The
results show that pressure calculation with the aid of Donnan equilibrium accurately describes
the conditions for a wide range of sodium concentrations and bentonite densities /17-16/.
Bentonite with a naturally high content of sodium chloride due to exposure to sea water has
been investigated in cooperation with Enresa in the Barra project /17-17/. The main purpose
of tests and analyses was to study the stability of the bentonite in connection with long-term
exposure to high salinities. Sodium chloride concentrations of more than two moles per litre
were measured in most analyzed samples. Swelling tests were performed with pure water after
desalination and with sodium chloride concentrations of up to three moles per litre. The results
showed pressure conditions that closely resembled those measured for MX-80 over a wide
range of densities.
Programme
Laboratory investigations of osmotic effects for other ionic species than sodium (mainly
calcium) have been commenced and will be finished during the coming period. The experimental
programme includes various bentonite materials as well as bentonite density and ion
concentration as variables. Swelling pressure and hydraulic conductivity are determined for
each individual ion equilibrium. A small number of tests will be conducted at elevated temperatures.
Development of the theory for Donnan equilibrium and the effects of swelling pressure
and ion concentrations in the buffer will continue.
The results of the laboratory tests are also expected to provide information on the pore structure
in the bentonite, which will be supplemented by special analyses of the test material, for
example by means of X-rays.
17.2.16 Ion exchange/sorption
Conclusions in RD&D 2001 and its review
SKB concluded that the physical properties of the buffer are greatly affected by the ion content
of the pore water. Under the chemical conditions that are expected to prevail in a deep repository,
it is above all the total salinity and exchange from sodium ions to calcium ions that can
influence these properties to an appreciable extent.
SKI maintains that SKB should evaluate what saline groundwaters mean for the repository.
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