Tracer migration in crystalline rock : application to geological barriers of nuclear waste storage

Abstract

This work deals with transport of radionuclides in the geosphere. The subject is investigated through characterisation of water conducting fractures, determining of rock transport properties and development of new methods for characterising geological samples. Here, as is often the case, radionuclide transport is investigated indirectly by characterising the structures, where the transport takes place, and directly by measuring transport properties in the gas phase. First water conducting fractures of three different types and the rock matrix surrounding them were analysed through rock samples from Olkiluoto. The analysis was done using X-ray tomography, the 14C-PMMA autoradiography technique and petrographical methods. The rock properties around the fractures were found to be highly heterogeneous, but some combining features were found. This experiment also showed the strength of combining different analysis methods. Second, a method for converting local porosities obtained through the 14C-PMMA autoradiography technique to fracture apertures was developed by analysing the radiation emitted from the 14CPMMA filled fracture by Monte Carlo simulations and comparing them to fracture apertures. A connection was found and the method was validated by manufacturing artificial glass plate samples with fractures of known apertures, and analysing the autoradiographic images obtained from them. The method needs further development before it can be used on real rock samples, but it was shown to be extremely promising in the studies conducted. Last, the transport properties of Olkiluoto rock samples were measured directly using argon pycnometry and through diffusion, permeability and advection - matrix diffusion measurements in the gas phase. The gas phase methods exclude sorption and other chemical and water-based processes and thus only give information about diffusion. They are, however, over 10 000 times faster, making them excellent for planning of water phase experiments and giving preliminary results. Gas phase methods are also very powerful tools when investigating the long term transport behaviour, as it would take far too long to run water phase experiments to the late part of the breakthrough curves. Correlations between porosity, diffusion coefficient and permeability were investigated and found to exist only between the latter two parameters. The mathematical model and measurement setup for advection - matrix diffusion measurements were vastly improved during these studies and are now effective tools for such a measurement. Overall this work managed to produce usable parameters for the safety assessment of the upcoming nuclear repository. The methods used were found to be suitable for the experiments, and they were developed further during the experiments.