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Solute transport in fractured rocks

Analysis of analytical solutions and determination of transport parameters

Time: Thu 2020-02-27 10.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm (English)

Subject area: Chemical Engineering

Doctoral student: Shuo Meng , Kemiteknik, Division of Nuclear Waste Engineering

Opponent: Dr. Klaus-Peter Kröhn, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH

Supervisor: Associate Professor Longcheng Liu, Kemiteknik; Professor Emeritus Ivars Neretnieks, Kemiteknik; Associate Professor Lanru Jing, Hållbar utveckling, miljövetenskap och teknik

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In order to facilitate the assessment of the safety and function of deep geological repositories for radioactive waste, several models have been developed to describe water flow and transport of solutes in fractured crystalline rock. The rock around the repository is described and modelled as a network of water-bearing fractures.

The first part of the work concerns analytical solutions of the mathematical models, first developed in the 1980s to describe nuclide transport with seeping water in the fractures where the nuclides can also diffuse in and out of the pores into the rock matrix. A new simple analytical solution is described in which the interaction between matrix diffusion and hydrodynamic dispersion could be decoupled, which makes the interaction between the processes visible while making the solution more manageable. In addition, another dispersion mechanism caused by the presence of independent transport paths is easily handled with the new model. This makes it possible to treat both dispersion mechanisms with the same formalism. This makes the new model more useful in interpreting field experiments with tracer as well as for long-term simulation of nuclide migration in rock.

The second part of the work is about molecular diffusion in the rock matrix itself, which is a central mechanism in the model above. One way to measure diffusion and sorption in rock pieces is to force ions through the pores of the rock by means of electromigration. The method previously used has been improved by adding a potentiostat and a pH buffer. The experimental results become more stable.

To better interpret the results, a general model for transport in the rock matrix was developed. The model includes electromigration, electroosmosis and dispersion in the pore system. The effective pore diffusivity and matrix formation factor can be determined from the experiments. The results show that the developed electromigration method can be used to provide high quality experimental data.