Diffusivity and porosity studies in rock matrix

Diffusion of radionuclides in the pores of rock adjacent to fractures is an important retardation mechanism in the far-field of the repository of nuclear waste. The nature of diffusivity and porosity in rock was studied as a function of various parameters. The phenomena of main interest were dead-end porosity, ion-exclusion and sorption. The rock types studied were rapakivi granite, granite and gneiss, tracer techniques was used as a research method. An analytical solution for outdiffusion from a porous cylinder was developed by applying a corrected form of Fick's second law for a case where part of the pores are so-called dead-end pores. With this model the theoretical curve could be closely fitted to the measured values. Matrix diffusion was studied in the nature in the island of Haestholmen on the coast of the Gulf of Finland, which due to postglacial land uplift started to rise from the Baltic Sea some 5000 years ago; the land uplift is some 3 mm per year. Rainwater has formed a layer of fresh groundwater floating on top of the saline layer. The coarse- grained granite bedrock of Haestholmen has been investigated for suitability of an underground repository for reactor wastes