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Cinétiques de dissolution des cristaux dans les silicates fondus : contexte des verres nucléaires

Judith Fournier-Renaud 1
1 LCLT - Laboratoire d'Etudes du Comportement à Long Terme des matériaux de conditionnement
DE2D - Département de recherche sur les technologies pour l'enrichissement, le démantèlement et les déchets : DEN/DE2D
Abstract : In France, High-Level radioactive Wastes (HLW) coming from nuclear spent fuel treatment are confined at the atomic scale in an homogeneous sodium-borosilicate glass called the R7T7 glass. At different stages of the melting process performed at high temperature, crystallized phases which incorporate the radionuclides are temporarily formed before their dissolution. The formation conditions of these crystals have already been studied but, to date, few studies on their dissolution in nuclear glass melt have been performed. The objective of this thesis is the description of the kinetics and mechanisms of the crystals dissolution in the borosilicate melt.This study focuses on the dissolution of rare earth silicates with an apatite structure (Ca2Nd8(SiO4)6O2 type) in three borosilicate melt compositions. Experiments are conducted in isothermal mode and in closed system. The crystalline fractions are followed according to time thanks to the acquisition of SEM images mosaics coupled with image analysis. In addition, chemical profiles at the crystal/melt interfaces are acquired by microprobe.The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, usually applied to describe the crystallization in melt, is successfully applied to the crystalline fractions measured during the dissolution process. The application of this model for different temperatures of thermal treatment validates its effectiveness to describe the dissolution kinetics of rare earth silicates in borosilicate systems in static mode. Its application is furthermore extended to the crystals dissolution in more complex systems where different types of crystals are simultaneously present. This model allows to determine the mechanism limiting the crystals dissolution in borosilicate melts, i.e. the diffusion in the studied systems, the activation energy of the dissolution as well as the constants of dissolution and the characteristic durations of dissolution for each considered temperature.
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Submitted on : Friday, November 16, 2018 - 4:54:08 PM
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Judith Fournier-Renaud. Cinétiques de dissolution des cristaux dans les silicates fondus : contexte des verres nucléaires. Physique Nucléaire Théorique [nucl-th]. Université Montpellier, 2017. Français. ⟨NNT : 2017MONTS034⟩. ⟨tel-01925448⟩

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