Speciation of Actinides in Granite Subjected to Tracer Studies
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.
KeywordsEuropean Synchrotron Radiation Facility Nuclear Waste Repository Hard Rock Laboratory Fissure Wall Sample Pillar
We thank HASYLAB and the ESRF for awarding us beamtime. We also are grateful to E. Sobolla (KIT-INE) and Julijana Krbanjevic (PSI) for preparation of the ground and FIB-ed samples and J. Göttlicher (KIT) for expert improvement of this manuscript. We acknowledge financial support as a Joint Research Group between the German Helmholtz Society and the Russian Basic Research Foundation (HRJRG-011 and 07-03-92280-SIG_a).
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