Modelling iron–clay interactions in deep geological disposal conditions
Résumé
In the context of deep geological disposal of high level radioactive wastes, the interactions between iron and clay-rich materials may
lead to adverse transformations of clay minerals with a potential loss of confining properties such as swelling and capacity to exchange
cations. Such transformations have been experimentally observed at temperatures starting at ca. 80 C, where smectites contained in a
mixture of bentonite and iron powder are transformed into iron-rich serpentine-type minerals.
The reaction-transport code CRUNCH is used to investigate the iron–clay interactions at 50 C over a period of 10,000 years, which
are the conditions considered here to represent the mean temperature value and the expected timescale for the corrosion stage. The aim is
to predict the nature and quantity of corrosion product, calculate the chemistry of water (essentially the pH) and the mineralogical transformation
in the system containing the canister, an optional engineered barrier (bentonite) and the host-rock (argillite).
The results of the calculations show that at the interface with the canister, where steel corrosion occurs, the iron is partly immobilized
by the precipitation of iron oxides (essentially magnetite) and small amounts of siderite. The pH stabilizes at high values, between 10 and
11, at this location. In the bentonite or the argillite in contact with the container, the primary clay minerals are destabilized and iron-rich
serpentine-like minerals precipitate as observed in the experiments (cronstedtite and berthierine). These minerals show low cation
exchange and swelling capacities.
The results also show that the interactions between iron and clay may lead to significant porosity changes in the system. A reduction
of the porosity is predicted at the surface of the steel canister, due to the precipitation of iron oxides. Porosity increase is predicted in the
clay material due to the dissolution of the primary clay minerals. The effect of these porosity changes have yet to be observed experimentally
and assessed with regard to the confinement capability of the repository near field, especially by considering the mechanical
aspects in the case with and without the engineered barrier.
2006 Elsevier Ltd. All rights reserved.