The main channel through which the radionuclides stored can re-ascend from a deep geologic site implies dissolution and water transport. The quality of a site thus depends on the following.

• Poor water mobility within the site.

• The thickness and quality of the confinement layer.

• The solubility limits of the radioelements in the water present in the medium. In general, this solubility depends on the pH and on the mechan­isms that regulate the water chemistry in the medium.

• The sorption properties of the radioelements by the medium; these too depend on the pH and how the water chemistry is regulated by the medium.

Various geologic formations seem to have satisfactory properties as disposal sites. The most popular are clay, as discussed here, granite and salt. Values representative of water mobility in these three media are given in table I.4.

As the table shows, water mobility in granite is particularly low. However, granite is seldom free of faults through which water can infiltrate. It is thus important to find structures whose characteristic dimensions are sufficient to ensure satisfactory performance. As is shown in table I.4, these characteristic dimensions are as 16 to 1 compared with clay, leaving many possibilities open. Civil engineering works, thermal constraints due to the heat released by the wastes, and possible seismic events can have con­sequences that have to be evaluated.

In spite of its large diffusion coefficient, salt is often considered a particularly promising medium. Indeed, a salt bed can be durable only in the absence of water, making the value of the diffusion constant purely academic. The diffusion of radioelements in anhydrous salt is thus extremely slow. Moreover, as salt creeps easily, it should fill up the constructions on the site rather quickly. However, salt is and probably will continue to be of economic value. A decision to work the site after the memory of its use is lost is thus possible. In this case, water injected in the vein to retrieve the salt more quickly would be laden with radioelements since the diffusion constant is large. Finally, in the long term, invasion of the disposal site by water cannot be excluded.

Finally, clay seems to offer excellent capabilities, combining the qualities of salt and of granite: small diffusion constant in spite of water saturation, creep properties sufficient to ‘eradicate’ defects, and good sorption of a large number of elements.

In the case of normal operation of a representative geologic disposal site, the risks to the most exposed population remain extremely marginal at all times in the future. The main confining agent is the geologic formation. Its mobility, its solubility, and its several million years half-life make 129I the main potential agent causing irradiation to the critical populations. Almost all of this iodine should end up in the biosphere within several hundred thousand years. In contrast, the transuranic elements are very unlikely to contaminate the biosphere.

The reflections presented here rest on rather limited measurements of clay diffusion properties. A great deal of validation work remains to be done. The influence of temperature and mechanical constraints has to be ascertained. A thorough exploration of the site to obtain precise descriptions of the clay layer is needed. However, it seems that nuclear waste disposal can be considered a realistic and feasible option that should not bring on major risks to future generations.

Acknowledgments. The authors thank G Pepin and E Tevissen from ANDRA for providing the documents that served as the basis for this presentation and for the helpful information they supplied concerning the phenomena considered.