Concept for Numerical THM Integrity Analysis of a Generic Repository in Clayey Rock in Germany

Within the ANSICHT project, BGE Technology, BGR, and the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) developed a safety assessment methodology for a high-level radioactive waste repository in clay formations based on German legal requirements. This included the quantification of integrity criteria and their exemplary numerical evaluation by THM-modeling. In the project's second phase, the modeling concept was extended by a detailed near-field model and initial attempts were made to quantify uncertainty. For the safety assessment of a repository system, detailed knowledge of the processes in the nearfield, the processes in the host rock, and the evolution of the whole system up to the surface is equally important. Concerning the prediction period of 1 million years and the complex coupled processes triggered by the construction and operation of an underground repository, the computational effort for numerical modeling is enormous. To cope with this effort, the proposed concept consists of models with different resolutions, dimensions, considered processes, and local coverages: a detailed 3D model of the near-field, a 2D and a 3D model of the whole domain, with the 2D model allowing a much higher resolution. The associated mathematical models are solved using OpenGeoSys 6, a finite-element-simulator allowing fully implicit process couplings. A statement about the integrity criteria and the general THM behavior is made integrally by all three models. Furthermore, a simplified 2D model is used to quantify the effect of variations in material parameters on the uncertainty of the results with manageable effort. Various geological setups have been used to investigate the influence of large inhomogeneities, such as sand lenses or layers in meter- to kilometer-scale, on the integrity of the host rock.

In this contribution, the updated modeling concept is presented and applied illustratively to a generic but typical geological situation in northern Germany.