This programme is specially aimed at investigating the relationship between radiation effects, due to various particles emitted by alpha decay of uranium and during fission reactions, and changes in the structure of organic matter (OM) with which it is in contact. In the Oklo s.l. U-ore, it is intended to characterize the different types of (U + OM) phases, ranging from "invisible" U-rich OM to millimetric uraninites inclusions within the OM, in order to estimate the specific consequences of radiation damages.
Bitumens are used for the solidification of low activity alpha containing wastes and their long term behaviour in geological conditions remain questionable. It is thus proposed in this work to gain some understanding on this issue by studying natural analogues of technological bitumens which have been poorly investigated, from this viewpoint, until now. In particular, we intend to characterize the organic matter associated with the Oklo nuclear reactors (Gabon) and to estimate the specific consequences of radiation damages, notably radiolytic effects, by comparison with organic matter collected outside the reaction zones and having variable uranium concentrations. 10 samples of organic matter (OM) from Oklo have been analyzed by carbon-13 nuclear magnetic resonance (NMR) spectroscopy with the cross polarization magic angle spinning (CP/MAS) technique. This technique has the advantage of giving quantitative information on the structure of OM, their functionality and the distribution of aromatic carbons. The samples present high aromacity factors (Fa) corresponding to high evolved OM. Concerning ion implantations, one does not observe macroscopic degradation of OM. When followed by leaching the implanted areas seem to present a distinct dissolution behaviour, in being apparently more resistant to aqueous corrosion than the unimplanted areas. The Rutherford backscattering (RBS) spectra of the samples analyzed exhibit, both before and after leaching, a shape which is typical of heterogeneously distributed uranium.
It is thus proposed to gain understanding on the long-term behaviour in a deep geological repository of bitumenized low-activity alpha-containing wastes by studying analogues of technological bitumens.
The original experimental procedure relies on the characterization of radiation-induced structural and chemical modifications with a variety of physico-chemical techniques. On powdered samples, the Total Organic Carbon (TOC) is analyzed; 13C NMR and infrared spectroscopes will provide results which allow the different types of bitumens and kerogens to be discriminated. Infrared Spectroscopy and Raman microanalyses are appropriate techniques for the in-situ characterization of the OM during the radiolytic alteration process. These analyses, coupled with 13C/12C isotope data and ion images (in-situ microscale cartography) obtained by SIMS (ion microprobe), more accurate discrimination of the different OM samples selected for this study.
Another aspect of this programme is calibration of the radiation effect induced by increasing doses of well-defined particles (mass, energy) in order to interpret the direct characterization of samples naturally irradiated during the functioning of reactors. For comparison with Oklo, OM from the Lovède U-ore may constitute another good analogue as the U-rich mineralisation is younger (jurassic age) and, as at Oklo, was controlled by migrated OM.
Funding SchemeCSC - Cost-sharing contracts