Hydrothermal reactivity of mixed-layer kaolinite/smectite and implications for radioactive waste disposal

The most valuable properties of natural clayey materials used as backfilling in nuclear waste isolation programmes are (1) their high ability to retard the movement of radionuclides, and (2) their high expandability properties to establish an integrated canister/clay/rock contact. The reactivity of a clay material with dominant kaolinite/smectite mixed-layer (K/S) was tested under simulated disposal conditions, with deionized water and CaCl (sub 2) solutions at 150, 200 and 250 degrees C for 1 to 12 months. XRD and chemical analyses indicate that the K/S, initially 50 % kaolinite-50 % smectite with 15 % low-charge and 35 % high-charge smectite layers, reacts after 4 months alteration at 250 degrees C to produce a K/S with 10 % kaolinite-90 % low-charge smectite. At that time of reaction, high-charge smectite layers are fully converted into low-charge smectite layers. The direct consequence of this reaction is a marked increase in the cation-exchange capacity of the clay material. A second result is that addition of Ca (super 2+) to solutions increases the proportion of newly-formed low-charge smectite at the expense of high-charge smectite layers. Hence, the irreversible fixation of poorly hydrated cations (Na (super +) , K (super +) ) by the high-charge smectite layers during the initial period after waste disposal would be time delayed; this could preserve the long-term radionuclide fixation capacity of the backfilling material.