Improving fracture-flow models by experimental evidence from process tomography

Advective fluid flow transport controls the migration of radionuclides in fractured crystalline rocks. Thus, the
safety assessment of deep geological repositories in crystalline rocks relies critically on fracture flow properties
and the reliability of transport modelling approaches. Here, we focus on heterogeneity and complexity of transport
processes, typically of limited predictability. In order to tackle this issue, we suggest experimental observations by
using tomographic methods, as well as feedback with and improvement of existing transport modelling approaches.
As an example, tracer propagation through fractured crystalline rock cores from the Czech Republic (Bukov URL,
depth of 500 m below the surface), was studied in collaboration between HZDR (Germany) and UJV (Czech
Republic). Spatiotemporal data of the tracer concentration during conservative transport are based on positron
emission tomography (PET), and the underlying fracture structure was characterized by microCT-imaging. The latter
yields a structural model for reactive transport modelling. The PET data sequences provide (i) the validation of
existing simulation approaches, and (ii) serve as input or the parameterization of advanced simulation concepts.
First results underscore the outlined approach. In particular, the PET measurements clearly show preferential and
localized pathways, a feature of the process that significantly reduces the effect of interactions at the fracture
surface (and thus retention by adsorption); although repeat experiments are suggesting that the identified pathways
are not constant over the experimental periods.
As a consequence of the combined experimental and simulation approach, we expect (i) advanced model concepts
based on experimental insights and (ii) an improved understanding of reactive transport processes with a focus on
temporal heterogeneity of preferential pathways.