Laboratory studies of radionuclide transport in fractured crystalline rock

Laboratory migration experiments over a distance of 1 m in natural fractures in quarried blocks of crystalline rock in a dedicated facility are described. These experiments have evolved with time, from early ones carried out under relatively uncontrolled conditions, to present and planned experiments that incorporate hydrogeological investigations of the fracture and include control of the groundwater chemistry. Radionuclide migration experiments have been carried out using 3H, 85Sr, 95mTc, 131I, 137Cs and 144Ce. The extent of retardation is in general agreement with that predicted from data obtained from static sorption studies. Post-experimental analyses of the fracture surfaces by autoradiography and two-dimensional gamma scanning have been used to determine the distribution of radionuclides on these surfaces, and have confirmed the strong retardation of 144Ce near the injection borehole. Cores, drilled orthogonally from the fracture surface into the rock mass, have been used to determine the extent of radionuclide diffusion into the interconnected pore space of the rock. Future experiments are described that will involve 241Am-labelled polystyrene microspheres and radionuclide-labelled, naturally occurring colloids to study the migration behaviour of colloidal material. (author)