A repository in clay: GRS develops model for prediction of gas transport
Besides salt rock and granite, clay is one of the three types of rock that researchers are currently discussing as options for the disposal of high-active waste. Clay is interesting as host rock since it has good retaining properties and is able to absorb radionuclides. This so-called sorption can be put down to the electrically charged surfaces of the clay minerals. Countries such as Switzerland or France are currently planning to build repositories in clay. In Germany, too, there are clay formations that might be considered for hosting a repository.
EU project FORGE: How do gases in a repository behave?
For the FORGE (Fate of Repository Gases) project, which is sponsored by the EU, GRS has studied the behaviour of gases in a potential repository in clay. Gases form e.g. upon the corrosion of waste containers or the decomposition of organic matter, which is often a constituent of medium- and low-active waste (e.g. from medical applications).
What is known is that gases will try and find the most different routes to escape from the repository. A relatively large proportion of them is dissolved in the water that is present in the pores of the clay and of other materials. Here, the gas is dissolved according to the same principle as carbon dioxide is dissolved in mineral water. In dissolved form, the gas can escape from the repository by so-called diffusion. Diffusion describes the accidental movement of the molecules dissolved in the water.
The task of GRS was to develop models that could be used to predict the transport of gases in a repository. For many years, the Swiss Nagra (National Cooperative for the Disposal of Radioactive Waste) has been carrying out experiments on gas transport in clay at the Mont Terri rock laboratory in Switzerland. The researchers at GRS evaluated and interpreted some of these experiments as part of the FORGE project. They wanted to know which models could be used to reproduce the experimental observations and whether this could help improve understanding the gas flow.
Is it possible to predict the behaviour of gas?
With these models, the researchers were able to show that the gas suddenly starts to flow through the clay as soon as the gas pressure exceeds the rock pressure. This is a clear indication of a mechanical opening of cracks. However, there was no water penetrating through these cracks – a phenomenon which according to the researchers can only be plausibly explained by very small crack openings.
What the studies of GRS also revealed, though, was that the crack-governed gas flow in the rock is a complex process that cannot yet be predicted in all aspects. One possibility for designing a repository in clay could therefore be to keep gas pressures as low as possible so that no cracks will form in the host rock in the first place. This would e.g. be the case if the gas could escape in a controlled manner along the engineered structures. A further possibility would be to slow down gas formation in general (e.g. by using certain waste containers or conditioning methods).
The results of the studies of GRS are presented in the report Modelling Gas and Water Flow through Dilating Pathways in Opalinus Clay - The HG-C and HG-D Experiments.