The site selection procedure in Germany attempts to find the most suited location for a deep geological repository (DGR) for high-level nuclear waste in either salt-, clay-, or crystalline-rock. For safety assessment, the modeling of coupled thermal-hydraulic-mechanical-chemical (THMC) processes in the far-field of potential repositories plays an important role, e.g. for quantification of potential radionuclide release through the geological barrier or for modeling large scale influences on the engineered repository hypothetically affecting its integrity.

In the BGE-financed project OpenWorkFlow (Lehmann et al., 2024), automated simulation workflows are developed, which shall support the safety assessments in the far-field, among others. These workflows

• take into account the specifics of the different host rocks,
• allow for selection of different THMC-process couplings for plausibility checks during development and for troubleshooting in the modeling workflow, and
• enable the modeling in 2D or 3D with meshes of varying geometric complexity.

Going beyond previous works (Silbermann et al., 2025; Zill et al., 2024), we realized a uni-directional coupling of reactive transport (C) to existing THM-coupled FEM models. For this purpose, we use the Darcy-velocity and other parameters from the THM-simulations to feed them into radionuclide transport simulations. This is valid, as long as there are no significant feedbacks from reactive transport on the THM-formulation via porosity- or fluid-density-alterations, among others, which is often the case.

A uni-directional coupling has the advantage that models for THM-processes and reactive transport can be set up and simulated independently from each other, while meshes are ideally identical or subsets of each other. This allows for separate modeling approaches for THM-processes and reactive transport, which can be optimized numerically for both models independently, which improves the accuracy and robustness of the individual simulations.

The developed workflows are applied to THM far-field models, where the model domain spans from bedrock to the ground surface. Based on this, radionuclide transport is computed on a subset of the model domain which focuses on the host rock to assess, if the potential radionuclide release through the geological barrier stays below the legal limits.