Daniela Schoenenbach oral presentation (OA1-Tue2-2-4)
Application of SIMS and Laser-SNMS to Study Actinide Diffusion in Natural Clay
Johannes Gutenberg University Mainz, Fritz-Strassmann-Weg 2, 55128 Mainz, Germany
Long-lived actinides like 237Np (t1/2 = 2.14 × 106 a) and 239Pu (t1/2 = 2.411 × 104 a) are of major concern for the storage of spent nuclear fuels due to their radiotoxicity. To evaluate the suitability of possible host rocks, like argillaceous rock, for a future repository, it is crucial to investigate interactions, such as sorption and diffusion, of the afore-mentioned radionuclides with the respective host rock.
Migration studies require analytical methods with high spatial resolution, such as Secondary Ion Mass Spectrometry (SIMS). However, these studies are carried out at environmentally relevant concentrations of the investigated radionuclides. This fact often challenges the detection limits of traditional SIMS. Therefore, a combination of SIMS with resonance ionization is pursued, where the secondary neutrals are ionized via multi-step laser excitation providing higher sensitivity and selectivity (Laser Secondary Neutral Mass Spectrometry, Laser-SNMS).
SIMS was applied to study the diffusion of 8 µM 237Np(V) in Opalinus Clay (OPA, Mont Terri, Switzerland). OPA is a heterogeneous, natural material consisting mainly of sheet silicates (66 %), quartz (14 %), calcite (13 %) and iron(II)-bearing minerals (4 %) . This fact can be observed and displayed on a μm scale using SIMS. Figure 1 shows different mappings of a 500 x 500 μm2 section of the afore-mentioned diffusion sample. The mappings a), b) and c) hereby refer to element masses that can be associated to sheet silicates (a), calcite (b) and pyrite (c) respectively. In addition to the spatial distribution of the clay constituents, it was possible to detect 237Np in this sample area using SIMS. The observed spatial correlation between Np and Fe (pyrite) (see Fig. 1 c), d)) agrees with a previous study using synchrotron-based micro-beam X-ray fluorescence . To increase the sensitivity for the detection of Np and other actinides, the experimental setup for Laser-SNMS and first results for Pu will be presented.
This work was financially supported by the Federal Ministry of Education and Research (BMBF) under contract No. 02NUK044B.
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