Sintering of MOX fuels : Influence of fabrication conditions and oxygen partial pressure

Les phénomènes de diffusion se produisant lors du frittage des oxydes mixtes d’uranium et de plutonium (MOX) dépendent du potentiel d’oxygène de l’atmosphère du four. L’atmosphère et la température engendrent des écarts à la stœchiométrie oxygène, appelée rapport O/M, dans l’oxyde mixte U1-yPuyO2±x. Ces travaux de thèse ont porté sur une meilleure connaissance de l’évolution du rapport O/M et de la microstructure des céramiques en fonction de la composition du mélange UO2/PuO2/(UPu)O2 et du potentiel d’oxygène au cours du frittage. Leurs effets sur la densification et les microstructures ont été étudiés, ce qui a permis de proposer la mise œuvre d’une poudre de solution solide (UPu)O2 obtenue par coprécipitation oxalique.Un suivi innovant de la PO2 a été conçu par un asservissement et des mesures entrée / sortie des fours par des pompes oxygène et sondes zircone. Ainsi, le rapport O/M a pu être suivi quantitativement tout au long d’un cycle de frittage. Il permet une bien meilleure compréhension des échanges d’oxygène en température entre l’échantillon et l’atmosphère. Un écart important entre les prévisions des calculs thermodynamiques et les données expérimentales est observé, même après plusieurs heures à 1700°C, écart particulièrement sensible à la formation de la solution solide (UPu)O2 ainsi qu’aux échanges locaux solide/gaz. Un schéma réactionnel de variation du rapport O/M au cours du temps, en faisant volontairement varier la PO2 du gaz a permis d’orienter les cycles de frittage pour obtenir la microstructure et le rapport final O/M souhaités.Ces nouvelles connaissances serviront de données d’entrée pour la modélisation de l’étape de frittage et ainsi de nouveaux cycles de frittage en rupture en four batch ou en four continu sont envisagés. Ils permettent d’obtenir de nouveaux produits et/ou des gains procédés. Ces avantages peuvent être couplés par l’apport bénéfique de la présence de solution solide (UPu)O2 en tant que nouvelle matière première.Diffusion phenomena occurring during the sintering of mixed uranium and plutonium oxides (MOX) depend on the oxygen potential of the furnace atmosphere. The atmosphere and the temperature generate deviations from the oxygen stoichiometry, called O/M ratio, in the mixed oxide U1-yPuyO2±x. The PhD work focused on a better understanding of the evolution of the O/M ratio and the ceramic microstructure as a function of the composition of the UO2 / PuO2 / (UPu)O2 mixture and of the oxygen potential during sintering. Their effects on densification and microstructures have been studied and have led to proposing the implementation of a (UPu)O2 solid solution powder obtained by oxalic coprecipitation.An innovative monitoring of PO2 has been designed by stabilizing and controlling O2 input/output measurements of the ovens by oxygen pumps and zirconia probes. It allows a much better understanding of the oxygen exchanges in temperatures between the sample and the atmosphere. A significant difference between the thermodynamic calculations and experimental data is observed, even after several hours at 1700°C. The kinetic effects are particularly sensitives to the formation of the (UPu)O2 solid solution as well as to local solid/gas exchanges. A reaction scheme for varying the O/M ratio over time, by voluntarily varying the PO2 of the input gas makes it possible to design sintering cycles in order to obtain the desired microstructure and final O/M ratio.This new knowledge will be used as input data for the modelling of the sintering step. New sintering cycles in batch oven or in continuous oven could then be considered. They make possible to obtain new products and / or process gains. These advantages can be coupled with the beneficial contribution of the presence of a solid solution as a new raw material

Sintering of MOX fuels : Influence of fabrication conditions and oxygen partial pressure

Les phénomènes de diffusion se produisant lors du frittage des oxydes mixtes d’uranium et de plutonium (MOX) dépendent du potentiel d’oxygène de l’atmosphère du four. L’atmosphère et la température engendrent des écarts à la stœchiométrie oxygène, appelée rapport O/M, dans l’oxyde mixte U1-yPuyO2±x. Ces travaux de thèse ont porté sur une meilleure connaissance de l’évolution du rapport O/M et de la microstructure des céramiques en fonction de la composition du mélange UO2/PuO2/(UPu)O2 et du potentiel d’oxygène au cours du frittage. Leurs effets sur la densification et les microstructures ont été étudiés, ce qui a permis de proposer la mise œuvre d’une poudre de solution solide (UPu)O2 obtenue par coprécipitation oxalique.Un suivi innovant de la PO2 a été conçu par un asservissement et des mesures entrée / sortie des fours par des pompes oxygène et sondes zircone. Ainsi, le rapport O/M a pu être suivi quantitativement tout au long d’un cycle de frittage. Il permet une bien meilleure compréhension des échanges d’oxygène en température entre l’échantillon et l’atmosphère. Un écart important entre les prévisions des calculs thermodynamiques et les données expérimentales est observé, même après plusieurs heures à 1700°C, écart particulièrement sensible à la formation de la solution solide (UPu)O2 ainsi qu’aux échanges locaux solide/gaz. Un schéma réactionnel de variation du rapport O/M au cours du temps, en faisant volontairement varier la PO2 du gaz a permis d’orienter les cycles de frittage pour obtenir la microstructure et le rapport final O/M souhaités.Ces nouvelles connaissances serviront de données d’entrée pour la modélisation de l’étape de frittage et ainsi de nouveaux cycles de frittage en rupture en four batch ou en four continu sont envisagés. Ils permettent d’obtenir de nouveaux produits et/ou des gains procédés. Ces avantages peuvent être couplés par l’apport bénéfique de la présence de solution solide (UPu)O2 en tant que nouvelle matière première.Diffusion phenomena occurring during the sintering of mixed uranium and plutonium oxides (MOX) depend on the oxygen potential of the furnace atmosphere. The atmosphere and the temperature generate deviations from the oxygen stoichiometry, called O/M ratio, in the mixed oxide U1-yPuyO2±x. The PhD work focused on a better understanding of the evolution of the O/M ratio and the ceramic microstructure as a function of the composition of the UO2 / PuO2 / (UPu)O2 mixture and of the oxygen potential during sintering. Their effects on densification and microstructures have been studied and have led to proposing the implementation of a (UPu)O2 solid solution powder obtained by oxalic coprecipitation.An innovative monitoring of PO2 has been designed by stabilizing and controlling O2 input/output measurements of the ovens by oxygen pumps and zirconia probes. It allows a much better understanding of the oxygen exchanges in temperatures between the sample and the atmosphere. A significant difference between the thermodynamic calculations and experimental data is observed, even after several hours at 1700°C. The kinetic effects are particularly sensitives to the formation of the (UPu)O2 solid solution as well as to local solid/gas exchanges. A reaction scheme for varying the O/M ratio over time, by voluntarily varying the PO2 of the input gas makes it possible to design sintering cycles in order to obtain the desired microstructure and final O/M ratio.This new knowledge will be used as input data for the modelling of the sintering step. New sintering cycles in batch oven or in continuous oven could then be considered. They make possible to obtain new products and / or process gains. These advantages can be coupled with the beneficial contribution of the presence of a solid solution as a new raw material

Développement d'un modèle multiphysique pour prendre en compte la variation de stœchiométrie en oxygène des combustibles MOX lors de leurs frittages

International audienceLes combustibles d’oxyde mixte d’uranium et plutonium doivent être frittés sous atmosphère hydrogénée pour être sous-stœchiométrique en oxygène. Pour une meilleure compréhension des phénomène ayant lieu lors du frittage, un modèle multiphysique est développé pour prédire la densification, l’évolution de stœchiométrie en oxygène et le champ de contrainte au sein du combustible lors d’un cycle de frittage

Assessing the oxygen stoichiometry during the sintering of (U, Pu)O2 fuel

International audienceDiffusion phenomena occurring in ceramics such as (U, Pu)O2 during sintering are affected by the oxygen content in the atmosphere. The latter sets the nature and the concentration of point defects which govern diffusion mechanisms in the bulk of the material. The oxygen partial pressure, pO2, of the sintering gas in equilibrium with mixed oxide (MOX) pellets needs to be precisely controlled; otherwise it may induce a large dispersion in the critical parameters for fuel manufacturing (Gauche, 2013; Matzke, 1987). It is crucial to understand the relation between the sintering atmosphere and the fuel throughout the thermal cycle. In this study, the oxygen potential of the sintering gas was monitored by measuring the oxygen partial pressure (pO2) at the outlet of a dilatometer by means of a zirconia probe. Coupling the thermal cycle with an outlet gas pO2 measurement makes it possible to identify different redox phenomena. Variations in the oxygen stoichiometry can be determined during the sintering of (U, Pu)O2, as well as can its final O/M. Our results make it possible to recommend a sintering atmosphere and sintering thermal cycle in order to obtain an O/M ratio that is as close as possible to the target value

Rheological study of binary mixtrures of powders using interparticular forces

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Predicting the flowability of alumina powder during batch grinding through the establishment of a grinding kinetic model

International audienceGrinding is one of the main unit operation in industrial processes handling powders. The particle size reduction that takes place during grinding tests, usually results in a significant change in the flow behavior of the ground powder. Up to now, a model predicting the evolution of powder flowability with grinding time, according to the operating conditions is still missing. In this paper, a methodology combining a grinding kinetic model and a flowability model involving the population-dependent granular Bond number is developed. The methodology has been applied to an alumina powder, ground in a batch ball mill. The flow function coefficient of the ground samples is measured after various grinding times in a powder shear tester. The comparison between model predictions and experimental data shows that this method allows an accurate prediction of the powder flow behavior over the first sixteen minutes of grinding

Predicting the flowability of powder mixtures from their single components properties through the multi-component population-dependent granular bond number; extension to ground powder mixtures

International audienceThe granular Bond number, defined as the ratio between interparticle attractive forces and particle's weight, can be computed to predict the flow behavior of powders. Previous studies used this dimensionless number to predict the flowability of various pharmaceutical or ceramic powders, exhibiting polydispersed particle size distributions. In this paper, we employ a multi-component population-dependent granular Bond number in order to apply this model to powder mixtures. Some binary and ternary mixtures are prepared using two different techniques: a Turbula® mixer and a ball mill. The flowability predictions appear to be in very good agreement with the empirical measurements, carried out with a powder shear tester. However, the model parameters seem to be slightly different between milled and raw powders. The model discussed in this paper allows a prediction of the flowability of powder mixtures according to their composition and serve as a guide for product formulation and equipment design

Investigating grinding mechanisms and scaling criteria in a ball mill by dimensional analysis

International audienceA dimensional analysis of the ball mill process is carried out through the Buckingham-Pi method. The dimensionless quantities identified are discussed and used in order to suggest scaling criteria for ball mills. The flowability and the particle size distribution of an alumina powder ground in laboratory ball mills of various dimensions are compared in order to discuss the influence and the relevance of each dimensionless numbers. Some geometrical, kinetics and dynamic similitudes are highlighted both theoretically and experimentally. In particular, the conservation of the Froude number and the fragmentation number lead to relevant scaling criteria for mills of 1, 2 and 7 L inner volumes. The importance of the ratio between the pebble size and the vessel diameter is also discussed. Finally, the preponderance of the fragmentation number over the number of revolutions of the vessel is interpreted in terms of particle fragmentation mechanisms

Multi-scale homogeneity analysis of co-milled powders : development of a reverse approach to assess quality of mixtures

International audienceThe standard methodology employed for estimating homogeneity of powder mixtures relies on the concept of scale of scrutiny. This parameter defines the relevant scale at which a homogeneous distribution of a given compound is critical regarding the final application of the blend. However, such a scale is not always known in advance. In this paper, a process involving two ceramic powders, co-ground and pressed into pellets is investigated in terms of homogeneity at various scales, from macroscopic to microstructural. According to the scale considered, different methodologies are employed. In particular, a reverse method is developed providing homogeneity indexes that are characterizing the microstructural state, without knowing the scale of scrutiny in advance. The results show that the homogeneity is improved by the co-grinding process as compared to simple drum-mixing experiments, even at a macroscopic scale. This method also shows that the microstructural homogeneity increases according to the grinding time during the first 16 minutes of grinding. The evolution of the microstructural homogeneity according to the grinding time is also investigated

Investigation of a granular Bond number based rheological model for polydispersed particulate systems

International audienceGranular materials are used in many industrial processes among various fields, such as pharmaceutical, food, metallurgy or nuclear fuel production. However, compared to other commonly used media, such as liquids, powders are known to behave unpredictably, leading to uncontrolled process operations. Since the flow behavior of the powders originates from interparticle forces, we suggest a model, linking the macroscopic flowability of powder beds, and the properties of the microscopic particles constituting the powder. A population dependent granular Bond number (Capece et al., 2016), that takes into account the particles properties such as the particles’ true density, surface energy, rugosity and the whole particle size distribution, is used. This non-dimensional number was found to correlate well with the flowability of polydispersed powder bed, which can be measured by shear testing with a Freeman FT4® powder rheometer. The results found in previous studies (Bernard-Granger et al., 2019; Capece et al., 2016) are extended and discussed using five different oxide powders exhibiting various flow behaviors. In particular, a short sensitivity analysis of the model is carried out. The results show that the fraction of fine particles within a polydispersed powder is a critical parameter for the flowability of the powder bed. Finally, the Rumpf’s theory is used to suggest a physical meaning for the model parameters