Validation and benchmarking of simplified reactive transport models of radionuclides for the assessment of nuclear waste repositories

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.