Characterisation of Opalinus Clay

Opalinus Clay is being investigated as a potential host rock for the storage of nuclear waste due to its very low hydraulic conductivity, self-sealing of open cracks and fissures, and excellent retention of radionuclides.

The image shows a Mont Terri collaborator in a gallery of the Mont Terri underground rock laboratory. In the foreground, there sits a 560 mm large diameter core of grey Opalinus Clay.
Opalinus clay core sample.

The embedding potential of Opalinus Clay is determined by its physical and chemical properties. Hence the principal interest is in its (im)permeability, capacity for self-sealing, and diffusion behaviour of radionuclides. If moisture penetrates open cracks, Opalinus Clay swells up, causing open fissures, like those that occur during the building of tunnels, to close up.

This self-sealing capacity reduces crack permeability and the rock can regain levels found in the previously undisturbed state. These correlations are being investigated in various experiments. The diffusion behaviour of radionuclides and the retention potential of Opalinus Clay are being investigated in experiment DR (diffusion/retention), for example. In a small borehole, an isolated section is saturated with water and injected with a specific amount of tracer material (e.g. tritium). After waiting more than a year, the small hole is bored out. Measurements of the new and larger core ascertain how far the tracer has penetrated into the rock (see photo below). In the process, ‘non-sticking’ radionuclides (e.g. tritium) spread more quickly than very ‘sticky’ radionuclides (e.g. caesium, cobalt) that will be present in deep storage facilities for radioactive waste. The diffusion of the radionuclides tested to date in these experiments in Opalinus Clay is well below the approved threshold.