The importance of Matrix diffusion for the transport of mobile radionuclides in granitic rock - summary of laboratory - scale experiments

Crystalline rock has been considered as a host medium for the repository of high radioactive spent nuclear fuel in Finland. Radionuclide transport take place along water-carrying fractures and matrix diffusion has been indicated to be an important mechanism, which will retard the transport of mobile fission and activation products. Transport properties of mobile radionuclides in crystalline rock have been studied by means of different laboratory-scale methods using rock-fracture and rock-core columns and a rock-block having a horizontally planar natural fracture (0.9 m x 0.9 m). Objective of this study was to examine the processes causing retention in solute transport through rock fractures, especially focused on the matrix diffusion. Results of this work can be used to estimate importance of matrix diffusion as a retention process during transport in different scales and flow conditions. Rock matrices of mica gneiss, unaltered, moderately altered and strongly altered tonalite and Kuru grey granite have been well characterised. Total porosity and the surface areas of mineral grains available for migration of species have been determined by the 14C-PMMA method. Pore apertures and geometry in mineral phases were analysed by field emission scanning electron microscopy (FESEM) and the minerals were quantified by energy dispersive X-ray micro analysis (EDX). The surface area of the solid rock was determined by the B.E.T. method. Hydrological properties of the columns or the natural fracture were characterised and flow paths were determined. Tracer experiments were performed using dye tracer uranin, and radioactive tracers HTO, 36Cl, 99Tc and 131I. Transport of tracers was modelled using modified migration models used in performance assessments, which were adapted for interpreting laboratory-scale experiments. The dominant matrix diffusion behaviour was demonstrated in porous ceramic columns. Demonstration of the effects of matrix diffusion in crystalline rock fracture column succeeded in a series of experiments where the experimental arrangements enabled very low water flow rates. Rock-block and rock-core column experiments could be modelled by applying consistent parameterization and transport processes. The effects of matrix diffusion were demonstrated on the slightly sorbing tracer breakthrough curves. Based on scoping calculations matrix diffusion begins to be significantly observable for a non-sorbing tracer when the flow rate is 0.1 μl.min-1 in the core column experiment and 1 μl.min-1 in the block experiment. This knowledge and understanding of the transport processes is transferable to different scales from laboratory to in situ conditions. (authors)