Radionuclide transport in fractured granite interface zones

In situ radionuclide migration experiments, followed by excavation and sample characterization, were conducted in a water-conducting shear zone at the Grimsel Test Site (GTS) in Switzerland to study migration paths of radionuclides in fractured granite. In this work, a micro-scale mapping technique was applied by interfacing laser ablation sampling with inductively coupled plasma-mass spectrometry (LA-ICP-MS) to detect the small scale (micron-range) distribution of actinides in the interface zones between fractures and the granitic rock matrix. Long-lived U, U, and Np were detected in flow channels, as well as in the diffusion accessible rock matrix, using the sensitive, feature-based mapping of the LA-ICP-MS technique. The retarded actinides are mainly located at the fracture walls and in the fine grained fracture filling material as well as within the immediately adjacent wallrock. The water-conducting fracture studied in this work is bounded on one side bymylonite and the other by granitic matrix regions. Actinides studied in this work did not penetrate into the mylonite side as much as into the granite matrix, most likely due to the lower porosity, the enhanced sorption capacity and the disturbed diffusion paths of themylonite region itself. Overall, themaximumpenetration depth detectedwith this technique for Np and uranium isotopes over the field experimental time scale of about 60 days was about 10 mmin thegraniticmatrix, illustrating the importanceofmatrixdiffusion in retarding radionuclide transport from the advective fractures. Laboratory tests and numerical modelling of radionuclide diffusion into granitic matrix was conducted to complement and help interpret the field results. 2008 Elsevier Ltd. All rights reserved.