The geological stratum or 'host rock' that will accommodate the radioactive waste, is the final barrier, acting as a last resort when the lines of defence represented by the conditioning, containers and engineered barrier have given way. Its role, on timescales of thousands of years, will be to further slow down the migration of the final phalanx of radioelements that remain radioactive.
To host the future waste disposal facility France has chosen a deep layer of clay in the Paris basin, formed approximately 155 million years ago and referred to as Callovo-Oxfordian. The proposed site of the waste disposal facility is on the boundary of the Meuse and Haute-Marne départements on the eastern edge of the Paris basin. The region is not rich in mineral resources like coal and oil that might cause mines to be dug there.
The qualities of argillite that make it suitable for the disposal of radioactive waste
From a geological point of view, the Paris basin is like a bowl filled with a succession of sedimentary layers, the oldest of which outcrop on the edges, and the most recent of which lie in the centre of the basin in Ile de France. Because of their plasticity these layers contain no fractures, or very few.
Callovo-Oxfordian consists mainly of argillite, a very fine-grained sedimentary rock formed at the bottom of seas and lakes. It is a rock known for its very low permeability (water runs through it at a rate of a few centimetres every 10,000 years) and its superb ability to retain chemical elements. These two properties delay and limit the movement of the radioactive atoms in waste.
At a disposal site the main enemy is water, which leaches materials. The argillite particles have spaces between them, pores where small quantities of water can be found, but the porosity (percentage of spaces) of argillite is low and the radius of the pores is less than one tenth of a micron. Consequently, water movement is absolutely minimal: argillite contains fossil water in its pores that has not moved since the rock was formed.
The chemical elements dissolved in it, whether they are radioactive or not, circulate mainly because of their own movement (by diffusion); they are not moved by currents of water because of the low permeability of the layer and because there are no significant differences in pressure. With diffusion, a slow movement mechanism, it would take thousands of years for them to be transferred to the environment***(*)***.
Molecules of water are trapped by microscopic crystals in the form of tiny clay mineral flakes. These flakes, which have electrically charged surfaces, retain certain elements such as the caesium and strontium in radioactive waste. The argillite also contains carbonates and quartzes, minerals that give it chemical stability, a certain amount of solidity and the ability to remove heat.
The stability of the chemical medium over time, no matter what disturbances occur due to the deterioration of the materials used for the disposal facility's engineered structures, guarantees that the confinement properties of the clay are preserved. The mechanical properties of the rock prevent hairline cracks and fractures from forming as a result of the underground facility being dug, and increasing the permeability of the rock near the drifts.
These qualities, added to the excellent retention capacity, compactness and low permeability of the deep clay formations of the Callovo-Oxfordian, make these formations a particularly suitable medium for hosting high-level long-lived waste.
The flow of water through the geological layers above and below the Callovo-Oxfordian clay layer has also been studied. Water moves through these layers at the rate of a few metres per century at most.
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