Combining innovative experimental approaches and cross-scale reactive transport modelling for assessing coupled hydrogeochemical processes at interfaces in deep geological repositories for radioactive waste
Authors: Poonoosamy Jenna , Klinkenberg Martina , Lönartz Mara , Yang Yuankai , Deissmann Guido , Brandt Felix , Bosbach Dirk 
Published: November 2021

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.