Combining innovative experimental approaches and cross-scale reactive transport modelling for assessing coupled hydrogeochemical processes at interfaces in deep geological repositories for radioactive waste

Abstract

Abstract. Deep geological repositories with a multi-barrier concept are foreseen by various countries for the disposal of high-level radioactive waste. A reliable and consistent assessment of the safety of these repositories over time scales of some hundred thousand years requires an advancement of process understanding. Simulation tools need to be developed for a close-to-reality description of repository evolution scenarios. This is especially required to resolve the challenging task of comparing and assessing the safety of different repository concepts in different host rocks within the German site-selection process. The construction of underground galleries and geotechnical barriers in the host rock formation and the emplacement of nuclear waste packages will create perturbations induced by chemical, thermal and pressure gradients at the interfaces of the different barriers, leading to mineral dissolution and precipitation to achieve re-equilibration. Such coupled hydrogeochemical processes generate non-linear responses in transport and mechanical properties of barrier materials and host rocks, which have to be taken into account for a more rigorous assessment of repository system evolution. Reactive transport modeling (RTM) can be applied to investigate these perturbations and processes across temporal and spatial scales, from the micro-scale at interfaces via the repository near field to the entire repository system – information not accessible through experiments alone. Although RTM is capable of addressing highly complex hydrogeochemical phenomena, the application of RTM codes to real systems is impeded by the often simplified description of coupled processes. To enhance the predictive capabilities of reactive transport models and to gain fundamental insights into the coupling between solute and radionuclide transport properties (e.g., permeability and diffusivity) of porous media and dissolution/precipitation processes, we conducted experiments on “simplified” chemical systems combined with pore-scale and continuum-scale reactive transport modelling to study processes in isolation, with the final aim of improving conceptual approaches for process couplings implemented in reactive transport codes. In this context, we investigated the effects of coupled mineral dissolution and precipitation in porous media on changes in permeability using flow-through experiments conducted in a magnetic resonance imaging scanner, which enabled the in situ investigation of porosity evolution in combination with monitoring changes in permeability and mineralogy. Our observations showed that classical implementations in reactive transport codes such as the Kozeny–Carman equation (Carman, 1937) failed to reproduce the changes in permeability and that more sophisticated approaches are required (Poonoosamy et al., 2020a, b). Moreover, we developed a novel “lab-on-a-chip” setup, i.e., micronized counter diffusion reactors with in operando 3D Raman tomography (Poonoosamy et al., 2019, 2020c), which enables evaluation of the alteration in pore architecture and study of the effect of coupled mineral dissolution and precipitation on the diffusive transport of solutes and radionuclides in porous media. Our approach enables the development of process-based theoretical models which allow for improvements in RTM codes and for predicting the evolution of perturbed interfaces in waste repositories, thus building confidence in the predictive capabilities of reactive transport models and reducing uncertainties with respect to future repository evolution.