Estudio de la interacción del uranio (VI) con magnetita en medio bicarbonato. Efecto de la presión de hidrógeno

Résumé

The management of the high level radioactive waste supposes one of the greatest technological challenges for our society, considering the disposal in a Deep Geological Repository as the solution more viable for the management of these wastes. The aim of this repository is to isolate the radioactive waste from human being and the environment, until the disappearance of the radiological risk. In the high level nuclear waste repository concept, spent nuclear fuel is encapsulated in steel canisters which represent the first physical barrier to radionuclide migration. The contact of the groundwater with the canister might produce the anoxic corrosion of the iron after thousands of years and the release of radionuclides could occur. Magnetite is the most important end member of iron corrosion products under reducing medium, which is the condition expected in a deep geological high level radioactive waste disposal. For this reason, the study of the radionuclide retention role of magnetite is an issue of crucial importance for the performance assessment of spent nuclear fuel iron-containing canisters. Since the uranium dioxide is the main component of spent nuclear fuel, in this work it has been investigated the interaction of magnetite (commercial and generated in situ in the steel surface) with uranium(VI) in bicarbonate medium and at different hydrogen atmosphere. The evolution of uranium concentration in solution is determined by means of Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and the uranium sorbed onto magnetite has been characterized by using different analytic techniques such as Scanning Electron Microscope with Energy Dispersive Spectrometry (SEM-EDS), X-Ray Diffraction (XRD) and spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS) and Extended X-ray Absorption Fine Structure (EXAFS). The main conclusions obtained from this work are that magnetite displays a great capacity of elimination of aqueous uranium in bicarbonate hydrogen medium. This last one participates actively in the reduction on aqueous uranium in the presence of the magnetite. Additionally, the rate of the aqueous uranium elimination increases with the increment of the hydrogen pressure. The process of reduction of U(VI) in the surface of the magnetite was indicated by means of XPS and it was confirmed by XAS. In the experiments of interaction of U(VI) with the magnetite, the measured uranium concentration agree with the solubility of amorphous UO2(s), which was detected indeed in the surface of the magnetite generated in situ in the steel surface by means of DRX. In that way, there is an effective reduction process of U(VI).