Impact of iron corrosion on the mineralogical evolution of Callovo-Oxfordian claystone: in situ MCO experiment

Abstract The MCO experiment was carried out over a period of 12 years in the Underground Research Laboratory (URL) of Andra to investigate long-term effects of iron-claystone interactions. The test chamber consisted of two cylinders: the external cylinder (EC) was made up of Callovo-Oxfordian claystone (COx); the internal cylinder (IC) was a mixture of crushed COx and iron grains. A central heater system and water seepage simulated the heat released by the radioactive decay and the circulation of clay porewater respectively. A multi-technique characterization indicates that the EC showed very little changes in comparison with the pristine claystone. The clay particles of IC become enriched in Fe 3+ and evolve towards a composition close to odinite/berthierine. Several corrosion products are identified but no magnetite is observed, although this mineral is thought to be at the origin of the passivation effect playing a role in the corrosion rate and porosity clogging. The released iron remains in the IC and migrates up to the interface between the two cylinders. This underlines that the effect of iron corrosion on the surrounding host-rock is restrained to a small portion of COx, whereas the rest of the rock preserve its properties suitable for a HLW repository.