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Etude de l'effet de la radiolyse de l'eau sur la livixation de la zirconolite

Abstract : Zirconolite is one of the matrixes foreseen for the confinement of minor actinides in case of deep geological disposal. Indeed, zirconolite (general formula: CaZrxTi3-xO7 (0.8 < x < 1.37)) is able to incorporate rare earth elements and actinides by substitution in calcium and zirconium sites and, moreover, its chemical durability into water is well known. However, in case of deep geological disposal, after a long period, water can reach the confinement matrix and can be radiolysed at the moment of the radionuclide alpha decays. In this work we have thus studied the effects of water radiolysis induced by charged particles (alphas or protons) on the dissolution of a synthetic sintered zirconolite. The formula of this zirconolite is Ca0,8Nd0,2ZrTi1,8Al0,2O7 where Nd simulates the presence of trivalent and tetravalent actinides. We performed the irradiations with external ion beams in two distinct geometries where the fluences ranged from 1015 to 1016 ions.cm-2. In the first geometry the beam stops into water before the surface/water interface. In the second one the beam gets through the sample before stopping at the surface/water interface. The use of these different configurations allows to study the respective influence of parameters such as sample irradiation, Linear Energy Transfer at the surface/water interface or total deposited energy. The irradiations were performed on both crystalline and amorphous zirconolites in pure water or with complexing species such as F-. The sample dissolution has been monitored through the release of cations. The radiolytic production of H2O2 has also been measured. Our results show that the water radiolysis has an effect on the preferential release of Zr, Ti and Nd: for these elements, releases are one or two order of magnitude higher than releases out of radiolysis. Such preferential releases occur whatever the temperature (20 or 50 °C), the surface state (crystalline or amorphous) and the experimental conditions (solid irradiation or not). The behaviour of Ti and Zr is enhanced (about 100 times) in presence of complexing species such as fluoride ions. Nevertheless, the alteration remains very weak, even in presence of complexing species and is sorely observable on the solid surface. Elementary releases are first kinetically controlled: if we base on Nd releases, the dissolution under radiolytic conditions is about 5 orders of magnitude higher than out of radiolysis. However, this kinetic step is shorter, a couple of hours, compared to a couple of days out of radiolysis. When there is only water radiolysis, a thermodynamic step is then observed as soon as the leachate is saturated in respect to the Zr and Ti hydroxides phases: elemental releases then become constant. When the solid is also irradiated, colloids are observed, although the leachate isn't saturated in respect with hydroxides phases. The formation of these colloids could arise from chemical equilibriums modification induced by ionisations occurring in the solid during its irradiation. The radiolytic production of H2O2 is lower than predicted by the calculated primary yield, because of the consumption of H2O2 or its precursors by chemical reactions involving others radiolytic species such as H2 or the aqueous electron.
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Submitted on : Tuesday, December 4, 2007 - 10:29:41 AM
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Magaly Tribet. Etude de l'effet de la radiolyse de l'eau sur la livixation de la zirconolite. Matériaux. Université Claude Bernard - Lyon I, 2007. Français. ⟨tel-00193589⟩

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