Non-destructive tomographic monitoring of transport processes in barrier material (Opalinus clay) with PET

Parameterizing transport in barrier materials is a challenge, because the processes are extremely slow, limited to smallest quantities, and frequently strongly localized, e.g. to fractures. These processes are generally well characterized on the molecular scale, but strongly affected by structural effects on the larger scales. Due to the intricate derivation of experimentally substantiated parameters, the impact of these scaling effects is often unduly neglected in process simulations for safety assessment.
As most sensitive tomographical modality, which is capable to monitor traces with molecular concentrations on macroscopic samples, we apply positron-emission-tomography (PET) with a high-resolution scanner („GeoPET“) for parameterizing transport in barrier materials (Kulenkampff et al. 2016a).
We focus here on diffusion in Opalinus clay as potential barrier rock for nuclear waste deposits (Kulenkampff et al. 2016b, 2016c). Our method is complementary to diffusion experiments in small diffusion cells and additionally provides information on heterogeneity and anisotropy of the process.
We derived anisotropic diffusion coefficients from the measured spatiotemporal tracer distribution which are in accordance with results from diffusion cells (Lippmann-Pipke et al., 2016). The spatial characteristic of the tracer distribution suggests that this anisotropy is caused by preferential transport along fine layers on the millimetre to centimetre scale. This finding should be considered in process simulations, because it means a reduction of the volume that effectively is affected by the process and thus faster progress of the tracer and a reduction of the reactive internal surface area, when adsorption is considered.
Other examples, where we take advantage from the favourable features of the GeoPET-method, are advective fluid transport in fractured salt and crystalline rocks, as well as reactive injection of water glass into salt rock.
In all these cases we monitor tracer concentrations and thus the key parameter for reactive transport modelling. We recommend GeoPET as unique experimental method to verify transport simulations on the macroscopic scale of drill cores.

Kulenkampff, J., Gründig, M., Zakhnini, A., and Lippmann-Pipke, J.: Geoscientific process monitoring with positron emission tomography (GeoPET), Solid Earth, 7, 1217-1231, 2016a.
Kulenkampff, J., Zakhnini, A., Gründig, M., and Lippmann-Pipke, J.: Quantitative experimental monitoring of molecular diffusion in clay with positron emission tomography, Solid Earth, 7, 1207-1215, 2016b.
Kulenkampff, J., Gründig, M., Zakhnini, A., Lippmann-Pipke, J.: Observation of 22Na+ - Diffusion in Opalinus Clay using Positron Emission Tomography (GeoPET) (mpeg-movie), https://doi.org/10.5281/zenodo.166509, 2016c.
Lippmann-Pipke, J., Gerasch, R., Schikora, J., and Kulenkampff, J.: Benchmarking PET for geoscientific applications: 3D quantitative diffusion coefficient estimation in clay rock, Comput. Geosci., in review, 2016.