Thermal integrity of clay and claystone – experiment and coupled THMC simulations

High-level radioactive waste generates heat. This is due to the decay of radioactive nuclei and nuclear physics processes in which the energy released by decay is converted into heat. In a repository, this decay heat is transferred from the storage casks to the surrounding claystone, which also heats up over time. The maximum temperature will be reached about 1,000 years after emplacement. According to the current requirements of the Repository Site Selection Act, a temperature of 100 degrees Celsius shall not be exceeded on the waste storage cask surface. The legislator has set this limit as a precautionary measure until further research results are available.

In one of their current research projects, scientists of GRS's Repository Research Centre are now investigating how claystone is influenced by the heat generation in a repository. In the project financed by the Bundesgesellschaft für Endlagerung (BGE), which is the federally-owned company for radioactive waste disposal, the researchers are approaching the topic both through laboratory experiments and numerical simulations.

Why claystone for a repository?

Claystone can be considered as host rock for a repository for various reasons. It is characterised by the fact that radionuclides adhere to it. Experts refer to this as sorption, which is due to the electrically charged surfaces of the clay minerals. Clay also has a very low permeability. Not least because of these properties, countries such as Switzerland or France are planning to construct repositories for high-level radioactive waste in claystone. According to studies carried out by the Federal Institute for Geosciences and Natural Resources (BGR), potentially suitable clay rock formations also exist in northern and southern Germany.

Concepts for generic repository sites in argillaceous (clay) rock envisage the storage of the waste at a depth of about 700 to 800 metres. According to these concepts, the storage casks holding the radioactive waste could be emplaced in drifts or in boreholes in a layer of claystone having a thickness of about 110 to 130 metres (southern Germany) or 500 to 600 metres (northern Germany). Currently, the idea is that the space around and between the storage casks and the claystone or the walls and ceiling of the drift would be filled with bentonite. Bentonite is a special type of claystone with a particularly high proportion of swelling and sorbing minerals.

Step 1: Reproducing processes in the repository by way of laboratory experiments

The experts already know a great deal about the processes in claystone from previous research projects. For example, the heat generated by the waste triggers various developments: Among other things, the clay expands strongly, which reduces both the pores in the claystone and the flow of liquids. At the same time, the pressure in the rock formation increases. In technical language, all these processes, which influence each other, are called "thermal-hydraulic-mechanical-chemical processes" – in short: THMC.

In the current project, the scientists now want to determine even more precisely how the claystone behaves at different temperatures in a range between 25 and 200 degrees. They are investigating both the claystone and the bentonite used for sealing. The aim is to find out whether the changes in the clay rock play a role in the so-called "integrity" of the host rock. This means that the various processes in the repository do not alter the claystone in a way that it will lose its sealing and containing properties. A loss of integrity could occur, for example, if the containers became leaky after a few thousand to ten thousand years and, due to excess pressure, a path were formed in the claystone through which radioactive substances from the waste could enter the overlying water-bearing rock strata.

To this end, the researchers are investigating the coupled thermal-hydraulic-mechanical processes by measuring in special containers in the laboratory how permeability and pressure change due to the swelling of the clay at rising temperatures. They produce the equipment they need to do so themselves in a 3D printer or have it made especially for a single experiment.

Step 2: Recalculate experiments with simulation codes

The repository researchers at GRS then use the results from the experiments for the development of simulation codes. Only when a process has been fully understood can a model for a code be derived from it.

While investigations in the laboratory can only be carried out selectively, simulation allows an insight into all possible states (e.g. at different temperatures and at different times). The project uses the open source code Phreeq. The thermodynamic reference database THEREDA is used as the database for the geochemical model calculations. THEREDA is being jointly developed and maintained by different research institutions. This project is also financed by the Bundesgesellschaft für Endlagerung (BGE).