β/γ-Autoradiography and collimated γ-spectrometry were applied on in situ impregnated core samples containing various radionuclides such as ¹³⁷Cs and ⁶⁰Co to study diffusion paths in fractured granitic rocks from the Grimsel test site (GTS). The radionuclide tracers were injected into a water conducting feature (WCF) during years (¹³⁷Cs) or months (⁶⁰Co) before the flow field and the adjacent rock matrix were overcored for analysis.
The retardation sites of the radionuclides in and around the flow paths were first determined by means of a state of the art β/γ-autoradiography scanner. Increased activity above natural background was observed both in the WCF and in the adjacent rock matrix showing structural control of radionuclide transport into the rock matrix as activity in the matrix was mainly bound to grain-boundary pores. In a second step, the areas of increased activity within the WCF and in the neighbouring rock matrix were investigated using a HPGe γ-spectrometer. A new setup for collimated γ-spectrometry was developed, which revealed spatial and nuclide specific information about tracer concentrations in the samples. Although this technique is hampered by much reduced counting efficiencies, it allows non-invasive determination of spatial distribution of radionuclides at the centimetre scale.
A preliminary data set produced to show the application possibilities of these techniques indicated maximum ¹³⁷Cs diffusion into the rock matrix adjacent to a WCF to a depth of 45 mm within 3 years and for ⁶⁰Co to about 3 mm in 2 months.