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In order to assess sites for a deep geological repository for storing high-level nuclear waste safely in Germany, various numerical models and tools will be in use. For their interaction within one workflow, their reproducibility, and reliability version-controlled open-source solutions and careful documentation of model setups, results and verifications are of special value. However, spatially fully resolved models including all relevant physical and chemical processes are neither computationally feasible for large domains nor is the data typically available to parameterize such models. Thus, simplified models are crucial for the pre-assessment of possible sites to narrow down the list of suitable candidates for which detailed site investigations and fully resolved models will be done at a later stage. Still, the accuracy of these simplified models is of importance as the pre-assessment of suitable sites will be based on them. In this study, we compare the modelling capabilities of TransPyREnd, a one-dimensional transport code based on finite differences, specifically developed for the fast estimation of radionuclide transport by the German federal company for radioactive waste disposal (BGE), with OpenGeoSys, which is a modelling platform based on finite elements in up to three spatial dimensions. Both codes are used in the site selection procedure for the German nuclear waste repository. The comparison of the model results of TransPyREnd and OpenGeoSys is augmented by comparisons with an analytical solution for a homogeneous material. For the purpose of numerical benchmarking, we consider a geological profile located in southern Germany as an example where the hypothetical repository is located in a clay-stone formation. TransPyREnd and OpenGeoSys yield overall similar results. However, both codes use different discretizations which impact is the highest for strongly sorbing compounds, while the difference gets negligible for less sorbing and more diffusive compo

Finding the best-suited location for safely storing high-level nuclear waste is more than a scientific challenge for a country. In Germany, the legal requirements demand a safe containment of high-level nuclear waste for at least one million years within a deep geological repository. In this context, the site selection procedure demands a validated model to predict the reactive transport of radionuclides in layered subsurface formations with varying physical and chemical properties. In this work, we compare the modelling capabilities of TransPyREnd, which is a one-dimensional transport code based on finite differences, especially developed for the quick estimation of radionuclide transport, with an established geoscientific simulator OpenGeoSys, which is capable of coupled thermo-hydro-mechanical-chemical modelling based on finite elements in three spatial dimensions. Both codes are going to be used in the site selection procedure for the German nuclear waste repository. The example host rock formation for benchmarking is the Opalinus clay located in southern Germany, which is modelled using a simplified and preliminary parametrization. We enhance the validation analysis by also benchmarking against an analytical solution for a homogeneous material. The modelling results suggest that both TransPyREnd and OpenGeoSys are capable of consistently modelling the distribution of radionuclides within the geological barrier over long timescale. The discussion further goes to the applicability of different approaches for serving as tools in the site selection procedure, with minor differences in their predictive capabilities, limitations, and possible pitfalls to be avoided for the accurate prediction of radionuclide transport over geological times.

The Federal Company for Radioactive Waste Disposal (BGE mbH) is tasked with the selection of a site for a high-level radioactive waste repository in Germany in accordance with the Repository Site Selection Act. In September 2020, 90 areas with favorable geological conditions were identified as part of step 1 in phase 1 of the Site Selection Act. Representative preliminary safety analyses are to be carried out next  to support decisions on the question, which siting regions should undergo surface-based exploration. These safety analyses are supported by numerical simulations building on geoscientific and technical data. The models that are taken into account are associated with various sources of uncertainties. Addressing these uncertainties and the robustness of the decisions pertaining to sites and design choices is a central component of the site selection process. In that context, important research objectives are associated with the question of how uncertainty should be treated through the various data collection, modeling and decision-making processes of the site selection procedure, and how the robustness of the repository system should be improved. BGE, therefore, established an interdisciplinary research cluster to identify open questions and to address the gaps in knowledge in six complementary research projects. In this paper, we introduce the overall purpose and the five thematic groups that constitute this research cluster. We discuss the specific questions addressed as well as the proposed methodologies in the context of the challenges of the site selection process in Germany. Finally, some conclusions are drawn on the potential benefits of a large method-centered research cluster in terms of simulation data management.