Are all smectite clays suitable as “buffers”?

In recent time search for suitable long-lasting “buffer clays” for isolating canisters with highly radioactive waste is going on in many countries. In Sweden and some other countries montmorillonite-rich clays are presently considered as primary candidates but other clays may do as well. The commonly expected major long-term alteration of buffer smectites is conversion to non-expandable forms like illite and chlorite, which reduces the swelling pressure of the buffer and thereby its tightness and self-sealing potential. This matter has been examined in a large-scale 2 year Mock-up test and small-scale hydrothermal tests using a Czech clay “RMN” rich in smectite and iron. The Mock-up test simulating the so called KBS-3V concept with a “canister” temperature of 95 °C showed that the Ca montmorillonite clay with 17% Fe lost more than half of its swelling pressure and became about 10 times more permeable than untreated clay. As expected from predictions based on chemical codes the smectite content remained nearly unchanged while significant loss of swelling ability and some slight loss in tightness took place. Both were ascribed to some dissolution of the smectite and precipitation of iron compounds that cemented partly dehydrated particle aggregates together.