Mineralogical Evolution of a Claystone After Reaction with Iron Under Thermal Gradient
Abstract
The design of the repository for high-level nuclear waste (HLW) in France consists of a
multiple-barrier system including steel canisters in a clay host rock. The system will undergo temperature
variations in time and space, the heat source being the HLW within the canisters. The effect of a thermal
gradient in space on the Fe–claystone interaction was investigated here by applying a thermal gradient
(150300ºC and 80150ºC) to a mix of claystone, Fe, and an aqueous chloride solution over periods of 3
and 6 months. Following the reaction, the starting clay minerals (mostly illite and mixed-layer illitesmectite)
evolved toward chlorite, Fe-serpentine, Fe-saponite, mixed-layer chlorite-smectite, or mixedlayer
serpentine-smectite as a function of temperature. Iron corrosion made the medium basic and
reductive. Magnesium enrichment of clay minerals was observed in the hottest part of the experiment due
to Mg migration under the thermal gradient. Reaction progress was enhanced at the lowest temperatures,
compared to batch experiments.