A disposal of highly radioactive long-lived waste

Final disposal solutions exist for two of the five categories of radioactive waste: very low level waste and low- and intermediate-level short-lived waste, which together account for 90% of all waste by volume. It should be possible to find a solution for low-level long-lived waste. Attention therefore focuses on ultimate waste from the nuclear industry: high-level waste and intermediate-level long-lived waste, which are the most active and long-lasting categories, although only very small volumes are produced. The proposed solution is deep geological disposal in a purpose-built repository.

The containment barrier formed by a deep-lying geological formation combines with the two barriers respectively created by the waste package in which the radioactivity is confined and by the structure in which the waste package is placed. These barriers impede any migration by radioactive atoms. There is worldwide consensus regarding the need for such repositories at some point in the future. What types of ground are suitable for such repositories? How can we ensure that no radioactivity escapes for a very long time?

In a geological repository, an underground rock layer serves, if not as a strong-box, at least as an obstacle to movement by such waste, which includes spent fuel assemblies and containers of vitrified waste. In the light of research conducted at an underground laboratory in Bure, France has opted for a 160 million year-old, 150 m thick layer of clay deep underground in the Paris Basin as the site for its CIGEO repository, on the boundary between the Meuse and Haute-Marne departments.

The harmfulness of radioactive atoms should not be exaggerated. Apart from one fleeting instant when they emit a ray, they behave like any other atom. Buried far beneath our feet, they would be no more likely to make their way to the light of day than an ordinary atom in the ground. In the absence of human activity, the only natural vector that might expose buried atoms is water.

Clay is notable not only for its plasticity but also for its ability to retain water that enters it. Water becomes trapped inside clay’s flake-based structure in thin films that have not moved since the clay formed during the tertiary or Mesozoic period. Due to this compartmentalised structure, there is almost no circulation of water. Atoms imprisoned within this structure move only very slowly. It would be an extremely long time before any buried atoms returned to the living world.