Tomographic analysis of advective flow and diffusive flux toward improved migration predictability in host rocks for radioactive waste

The predictive power of numerical approaches for the analysis of flow fields, e.g. for  radionuclide migration, depends on the quality of the underlying pore network geometry. Validation of the obtained simulation results can only be performed with a limited number of methods. Positron emission tomography (PET) is a suitable technique that has been established in geomaterial sciences in recent years. The use of suitable radiotracers allows the analysis of advective transport and diffusive flux in a variety of complex porous materials. In addition to the visualization of time-resolved transport patterns, the statistical analysis of transport controlling parameters is currently in the focus of investigations using PET techniques.

Using potential host rock types with low permeability for underground radioactive waste repositories as examples, we have analyzed the heterogeneity of the flow field at laboratory scale.¹ Diagenetic and sedimentary components and their pore size distributions and pore network geometries are responsible for the flow field properties. The resulting generalized pore network geometries are used in digital rock models to calculate effective diffusivities, using a combined upscaling workflow for transport simulations from the nanometer to the micrometer scale.² For advective transport in fractured crystalline rocks, PET provides evidence for the influence of fracture wall geometries over a wide range of the length scale. Surface building blocks from nm to mm size are responsible for the observed changes in breakthrough curve behavior. Finally, another hot topic is the testing of reactive PET tracers for materials analysis. In addition to the use of conservative tracers described above, reactive tracers provide insight into the density of reactive surface sites in complex porous materials.

¹Bollermann, T.; Yuan, T.;  Kulenkampff, J.;  Stumpf, T.; Fischer, C., Pore network and solute flux pattern analysis towards improved predictability of diffusive transport in argillaceous host rocks. Chemical Geology 2022, 606, 120997.

²Yuan, T.; Fischer, C., The influence of sedimentary and diagenetic heterogeneity on the radionuclide diffusion in the sandy facies of the Opalinus Clay at the core scale. Applied Geochemistry 2022, 146, 105478.