Thermal-geological data for preliminary safety analyses of repository systems in German sedimentary host rocks – new results of the project ThermoBase

Preliminary safety analyses are part of the site selection procedure for a repository of heat-generating radioactive waste in Germany. The Federal Company for Radioactive Waste Disposal (BGE) is conducting representative preliminary safety analyses for defined potential sub-areas. The assessment of the long-term safety of potential repository systems for heat-generating waste in these areas requires knowledge of the temperature field as the initial temperature of the host rocks is crucial for determining the repository design and for analysing expected future developments with regard to the safe containment of radionuclids. In the joint research project ThermoBase commissioned by the BGE, the GFZ Helmholtz Centre for Geosciences (GFZ) and the Federal Institute for Geosciences and Natural Resources (BGR) focus on the temperature distribution in areas with sedimentary host rocks of rock salt and claystone. The present day temperature field in the subsurface can be described on the basis of borehole observations supplemented by numerical temperature models. This approach is subject to significant uncertainty in areas with structural differentiation and low data density. Therefore, structural and thermal data must be determined, and uncertainties in thermal property variations and thermal boundary conditions must be considered, to adequately represent the host rocks in the models.

To better characterize the temperature distribution in the sub-areas, 3D finite element meshes are being developed by the GFZ to represent the geological structure in great detail. Thermal properties of host rocks are being measured in the laboratory and derived from high-resolution geophysical and temperature logs, ensuring accurate parameterization of the models. Transient temperature boundary conditions are incorporated into the simulations to account for past climate variations, such as glacial and interglacial cycles, influencing the current temperature field. First results, like heat flow calculations and temperature maps for depths of interest, offering insights into the spatial variability of the thermal field and its implications for repository planning.

The BGR uses generic geological models to conduct statistical numerical analyses on the effect of parameter variations on the temperature distribution for typical geological situations with sedimentary host rocks. Subsequently, a heat-generating term representing the heat introduced by a repository is included in the models, and the impact of uncertainties in input parameters on the safety-relevant temperature development in the repository area is assessed. Additionally, we provide insights into the minor effects of permafrost during potential future cold phases.