Prédiction de l'évolution granulométrique et morphologique d'une poudre dans un four tournant
Résumé
The dry conversion process for making UO2 nuclear fuel pellets consists of two steps: hydrolysis of UF6 to UO2F2, followed by reducing pyrohydrolysis to UO2 in a rotary kiln. The physical characteristics (morphology, particle size distribution) of the powder obtained at the kiln end determine the final properties (sinterability, flow-ability, green strength). We developed a mathematical model describing the morphological evolution of the powder in the rotary kiln, which enables us to predict morphological characteristics of UO2 powder as functions of the processing conditions. Firstly, the powder flow in the kiln was modelled, including the exchanges between a dense phase (powder bed) and an airborne phase (particles showering down). An original feature of this model is to consider the effect of lifters for calculating the dynamic variables. Secondly, the phenomena responsible for the changes in morphology and grain size were identified and modelled. A population of fractal agglomerates was considered, whose number and size change due to Brownian and sedimentation agglomeration, sintering, breaking up, and chemical reactions. The model is based on population balances and the particle size distribution is divided into sections. Results of both dynamic and morphological calculations are compared to available measurements. Lastly, the influence of the different mechanisms of morphological evolution on the final size distribution is analysed.
Origine : Fichiers produits par l'(les) auteur(s)
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