Etude de la réactivité chimique entre les précurseurs lors de l'élaboration de verres nucléaires enrichis en molybdène

Abstract : Nuclear waste glasses are produced by chemical reactions between a solid waste (calcine) and a glassy precursor (glass frit) through a high-temperature vitrification process. The waste is first dried and calcined (to lose water and nitrogen respectively), then mixed with the glass frit. A succession of physicochemical processes of impregnation, diffusion, crystallization and dissolution is involved in order to incorporate the radioactive elements within the glassy network. These reactions, which are dependent on the precursor composition and the synthesis conditions, must be complete to ensure the homogeneity of the glass and to guarantee its long-term behavior. The aim of this work is to determine the chemical reactions between the precursors and to quantify the reaction kinetics in order to identify the processes responsible for their limitation. A simplified system consisting of a sodium-aluminum borosilicate glass frit and a calcine containing sodium nitrate and aluminum oxide (the principal oxides present in complex calcines) is progressively complexified to determine the influence of low solubility elements initially present in the fission product solutions to be vitrified. The cases of molybdenum and neodymium oxides are the focus of attention. The formation conditions (time, temperature) of crystalline molybdates (sodium, calcium) and aluminates (sodium, neodymium) and their range of stability in the calcines are determined. The dissolution kinetics of these phases in the glass frit is modeled. It is shown that the dissolution of molybdenum, as Na2MoO4, is controlled by the thermodynamic solubility of MoO3 in the glass. It is independent of the sodium aluminate dissolution. For both, Na2MoO4 and sodium aluminates, dissolution reactions present an Arrhenian behaviour and the activation energies are close to that of the viscous flow. This work also describes the formation mechanisms of intermediate phases which can lead to the crystallization of the "yellow phase" (enriched in molybdenum, alkali and alkaline-earth oxides) that can form in more complex glasses.
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Elodie Boué. Etude de la réactivité chimique entre les précurseurs lors de l'élaboration de verres nucléaires enrichis en molybdène. Matériaux. Université Paul Sabatier - Toulouse III, 2017. Français. ⟨NNT : 2017TOU30362⟩. ⟨tel-02286314⟩

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