Characterisation of pore space geometry by 14C-PMMA impregnation—development work for in situ studies

In Finland, as in many other countries, high-level radioactive waste is planned to be disposed of in deep-lying crystalline rock. There has been a tendency to play down the potential role of the geosphere as a safety barrier in repository performance assessment. Among other reasons, current uncertainties in transport pathway definition and pore space characterisation of crystalline rock can be mentioned. Repository safety evaluation today requires going from laboratory and surface-based field work to the underground repository level. Little is known about the changes to rock transport properties during sampling and decompression. Recent investigations using resin impregnation of the rock matrix at the Grimsel Test Site imply that non-conservative errors in calculated transport properties derived from laboratory data may reach factors of 2/3. This may lead to over-conservative estimates of limited matrix diffusion depths, which may grossly underestimate geosphere radionuclide retardation.Due to the potentially great significance of pore space characterisation to safety analysis calculations, it was decided to study the rock matrix characteristics using an alternative resin technique. During last decade, the polymethylmethacrylate (PMMA) method has been developed for characterising the porosity of low-permeable granitic rocks in the laboratory. Impregnation with 14C-labelled methylmethacrylate (14C-MMA) and autoradiography allow investigation of the spatial distribution of accessible porosity at the centimetric scale. Quantitative measurement of total or mineral-specific local porosities have been also obtained using image analysis tools. Electron microscopy examinations and mercury porosimetry measurements have provided detailed information of pore and fissure apertures.The developments of the PMMA method for in situ testing at the Grimsel Test Site are reported here. Grimsel granodiorite samples were impregnated with MMA using various prehandling and impregnation conditions, resulting in variations of the measured porosity values. Vacuum drying of studied samples increased the PMMA and porosity values of water saturation gravimetry. If the rock matrix cannot be dried in situ, the intrusion behaviour of MMA in water filled pores will be a key consideration for successful in situ impregnation.