Euradwaste '08 - EU Bookshop - Europa

nant exchangeable cation. This change in the interlayer charge, together with the ionic strength of
the pore water, has an impact on the swelling capacity of the bentonite.
In the framework of NF-PRO, laboratory experiments were performed to investigate the effect of a
thermal gradient on the bentonite pore fluid composition and the hydration of unsaturated bentonite.
A compacted bentonite column was heated to 100°C at the bottom end while diluted granite water
was injected at the top surface, which was kept at ambient temperature. After 7.6 years, the column
was dismantled and analysed. Experimental results indicate that carbonate behaviour is consistent
with the dissolution of calcite leading to oversaturation of the porewater with respect to gypsum,
which precipitates in the upper part of the column, the latter resulting in a decrease in sulphate. Observations
from these experiments confirm that pH buffering can be explained by dissolution/precipitation
reactions.
Advances in studies related to corrosion
In a geological repository, the container isolates the disposed waste from groundwater for a designated
period of time. Corrosion will affect the containment properties of the container. A variety of
corrosion experiments have been conducted under NF-PRO in which key issues were addressed related
to iron and steel corrosion rates and interactions between corrosion products and the bentonite
buffer bentonite properties. Work by NF-PRO involves investigations on interactions between bentonite
and corrosion products from carbon steel canisters, or in the case of the KBS-3 concept, copper
canisters with cast iron inserts, and the effects of these interactions on the properties and potential
effect on the performance of the engineered barrier system.
Various experiments were carried out to identify the nature of corrosion products that are formed at
the container-bentonite interface. These experiments have shown that magnetite, which is formed
through an intermediate Fe(OH)2 phase, is a prevailing corrosion product.
A range of iron corrosion experiments have been conducted by NF-PRO to obtain long-term corrosion
rate data. Experiments performed in bentonite water or slurry at different temperatures have
shown that, after an initial stage of enhanced corrosion, the corrosion rates decreases to values below
5 �m/year. This indicates that that the corrosion rate is controlled by the formation of a corrosion
product layer. In the long term, it seems that the corrosion rate is mainly controlled by the
properties of the corrosion products. The initial corrosion rate increases with higher temperatures
but slows down after steady-state has been established.
Advances in studies related to canister-bentonite interactions
An important part of work by NF-PRO involved questions related to the effects of copper and iron
corrosion products on the bentonite properties. Laboratory tests under different experimental conditions
indicate typical diffusion depths in compacted bentonite in the order of less than 1 mm for
copper canisters to 5 to 6 mm for steel canisters.
Corrosion experiments performed under NF-PRO have shown that the physical properties of the
bentonite are modified by increased iron concentrations in the bentonite. In particular, this may increase
hydraulic conductivity, possibly decrease swelling pressure and decreased cation exchange
capacity. In the long-term, the physical properties of the bentonite may be affected by the conversion
of montmorrilonite to non-swelling Fe-silicates (smectite). Results from corrosion experiments
were modelled with the reactive-transport code PHREEQC.
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