Speciation of Actinides in Granite Subjected to Tracer Studies

Abstract

Tomographic studies of granite from the Äspö Underground Laboratory in Sweden are performed to understand the observed release of natural uranium in column studies upon application of groundwater flow. X-ray phase-sensitive tomography images reconstructed from in-line X-ray phase contrasted radiographs were compared with scanning fluorescence tomography reconstructions. The latter are based on sinograms of X-ray emission lines recorded with spatial resolution on a nanometer scale for a granite rock containing U after being subjected to a radionuclide tracer experiment. The results show that the U distribution follows microfissures or pores in the granite. Naturally occurring Th is revealed to be intimately associated with what appears to be feldspar, suggesting its being present as a dopant ion in the mineral structure. Neptunium tracer was not found in a sample prepared using FIB lift-out techniques, although the presence of Np was identified in the original granite piece during screening experiments prior to FIB-ing. The Np-containing part of the sample broke off during the preparation. Although this case study was limited to only three samples, initial interpretation of results confirm that Th is less likely than U to become mobile in groundwater upon drilling and excavation of emplacement caverns and shafts for a nuclear waste repository in granite bedrock.