Étude thermodynamique et cinétique de nucléation primaire et de la croissance cristalline en vue de la modélisation de la précipitation du peroxyde d'uranium en lit fluidisé

Abstract : The uranium peroxide precipitation in fluidised bed patented by AREVA is currently tested on pilot scale reactor on different mine sites for industrial applications in the short term. The yellow cake produced on this way has very interesting qualities in terms of density, particle size and morphology for handing and transport. In addition, the low level of impurities present in the uranium peroxide is an important advantage for the rest of the manufacturing process fuel. The objective of this work is to determine the thermodynamics and kinetics of primary nucleation and crystal growth governing the uranium peroxide precipitation for a global modelling to stimulate the system behaviour. In order to determine the uranium peroxide solubility over a wide range of operating conditions corresponding to the industrial process conditions, a solubility model has been developed and optimized using experimental measurements. In this study, the solubility product has been identified which allows the calculation of the supersaturation, driving force of the precipitation which is an essential parameter to know in order to study the nucleation and crystal growth kinetics. The primary nucleation is investigated by a phenomenological approach which implements a method developed by Bertrand-Andrieu based on the use of a very fast reagents micromixing device (~1ms). Uranium peroxide primary nucleation kinetics follows the Volmer and Weber law. Furthermore it is shown that the primary nucleation depends on the sulphate concentration, the corresponding law incorporates this link with the BN term related to the interfacial tension and thus the quantity of sulphate absorbed. Crystal growth kinetics is also determined by a phenomenological approach. Due to the hydrogen ion production during the precipitation reaction, the crystal growth experimental study is performed by a pH-metric monitoring. Independent of the impeller speed, crystal growth is surface integration controlled with a spiral mechanism. It is shown that the uranium peroxide crystal growth law is first order with respect to the supersaturation. The kinetic parameter kg is dependent on pH and sulphate concentration, an empirical law function of the experimental conditions is then proposed for this parameter
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Submitted on : Thursday, March 29, 2018 - 12:14:42 PM
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Séverine Planteur. Étude thermodynamique et cinétique de nucléation primaire et de la croissance cristalline en vue de la modélisation de la précipitation du peroxyde d'uranium en lit fluidisé. Autre. Université de Lorraine, 2013. Français. ⟨NNT : 2013LORR0019⟩. ⟨tel-01749380⟩

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