Dissolution of uranium dioxide in nitric medium, towards a macroscopic model of reactors

International audienceDissolution is a key step in several industrial processes. It is especially a milestone of the head-end of manyhydrometallurgical processes. For example, in recycling of spent nuclear fuel, the solubilization of the chemical elementsis essential before performing the liquid-liquid extraction steps to separate reusable material and final waste. One of themost complex scenarios is that of heterogeneous autocatalytic reactions. Today, there are few satisfying models forthese cases due to a lack of comprehension of their mechanisms.We focus here on the dissolution of uranium dioxide in nitric medium. In order to propose optimized processes fordissolution, this study aims at better understanding the chemical, physico-chemical and hydrodynamic phenomena ofsuch reactions. This study is also part of a modeling approach aiming, on one hand, at expressing the intrinsic reactionrates and describing the physico-chemical phenomena at interfaces and, on the other hand, at developing a generalmodel for dissolution reactors.Optical microscopy observation confirmed the highly autocatalytic nature of the reaction and led to measurements, forthe very first time, of "true" chemical kinetics of the reaction. The acid attack of sintering-manufactured solids occursthrough preferential attack sites. It develops cracks in the solids that can lead to their cleavage. This inhomogeneousattack is made possible by the establishment of bubbling in the cracks which allows periodic renewal of the reagents andthus maintains the reaction within the cracks. This point is a key component of the mechanism: a strong link among thedevelopment of cracks, bubbling through the cracks, and overall dissolution kinetics is demonstrated in this work.A model coupling material balance to the structural evolution of the solid and liquid phase compositions, and taking intoaccount the interfacial transport is proposed. The simulations based on this model are close to the experimentalobservations, and allow to replicate the effect of various reaction parameters for the very first time, such as the reductionof overall kinetics when turbulence increases

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Étude cinétique et physico-chimique des phénomènes d’agglomération en vue de la modélisation de la précipitation oxalique dans l’industrie nucléaire

L'auteure a souhaité limiter l'accès aux membres de l'Enseignement supérieur français.During oxalic precipitation, three major mechanisms take place: nucleation, crystal growth and agglomeration. After the acquisition of the kinetic laws of nucleation and growth of neodymium oxalate and uranium oxalate by Andrieu (1999), the objective of this study is to determine the respective agglomeration kinetic laws. Determining the agglomeration kinetic law consists more specifically in determining a quantity called agglomeration kernel. This kernel is a measure of the frequency and efficiency of the collisions which occur between particles in the reactor. The agglomeration mechanism is complex as it is sensible to many parameters. The objective is to determine a kinetic law showing explicitly the parameters influencing the mechanism of agglomeration. Mathematical and experimental methods are firstly developed on an inactive compound (neodymium III) and then applied on uranium IV, a simulant of plutonium IV. Experiments are conducted in a perfectly mixed reactor, under operating conditions similar to the industrial ones. An original mathematical method is also developed to solve the population balance. To obtain a predictive kinetic model, it is essential to consider deviation from ideality for the calculation of supersaturation, through activity coefficients. After a thermodynamic study, the Bromley model (1973) is finally chosen to evaluate activity coefficients. The overall processing of our experimental data leads to the agglomeration kinetic laws of neodymium oxalate crystals and oxalate uranium IV over a wide range of operating conditions.Au cours de la précipitation oxalique, trois mécanismes majeurs ont lieu : la nucléation, la croissance cristalline et l’agglomération. Après l’acquisition des lois cinétiques de nucléation et croissance des oxalates de néodyme III et d’uranium IV par Andrieu (1999), l’objectif de cette étude est d’acquérir la loi d’agglomération des cristaux. L’acquisition de cette loi consiste plus précisément en la détermination d’une grandeur appelée noyau d’agglomération. Ce noyau est une mesure de la fréquence des collisions et de leur efficacité. Le mécanisme d’agglomération est complexe car soumis à de nombreuses influences. L’objectif est de déterminer une loi faisant apparaître de manière explicite les paramètres influençant le mécanisme d’agglomération. Les méthodes expérimentales et mathématiques sont préalablement mises au point sur un simulant inactif (néodyme III) puis appliquées en actif sur l’uranium IV, simulant du plutonium IV. Les expériences sont menées dans un réacteur continu de type parfaitement agité, dans des conditions opératoires similaires aux conditions industrielles. Une méthode mathématique originale est également développée afin de résoudre le bilan de population. Pour obtenir un modèle cinétique prédictif, il est essentiel de tenir compte des effets de milieu pour le calcul de la sursaturation, au travers des coefficients d’activité. L’étude thermodynamique menée en parallèle des expérimentations nous a conduits à retenir le modèle de Bromley (1973) pour déterminer les coefficients d’activité. Le traitement global des données permet aujourd’hui de proposer une loi d’agglomération pour l’oxalate de néodyme et l’oxalate d’uranium IV sur un large domaine de conditions opératoires

MESURE DE MASSES DE NOYAUX DANS LA REGION N = Z = 40

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Physical simulation of precipitation of radioactive element oxalates by using the harmless neodymium oxalate for studying the agglomeration phenomena

International audienceOxalic precipitation is usually applied in nuclear industry to process radioactive wastes or to recover actinides from a multicomponent solution. This paper deals with the development of methods adapted to a nuclear environment in order to study the agglomeration phenomena during actinide oxalic precipitation. These methods are previously setup with harmless elements that simulate the actinide behaviour: the lanthanides. A parametric study is carried out to quantify the influence of operating parameters on the agglomeration kernel and to determine a kinetic law for this mechanism. The experimental study is performed in a continuous-MSMPR precipitator at steady-state. The method is based on the resolution of two population balances using the moment approach, one for elementary crystals and the other for agglomerates. Provided that the kinetic rates of nucleation and growth are known, the agglomeration kernel can be obtained from a mathematical treatment of the experimental particle size distributions. Results point out that experimental crystal sizes are consistent with an independent kernel. It appears that the agglomeration kernel is directly proportional to supersaturation, increases with temperature but is limited by ionic strength and shear rate

Relationship between the microstructure of MOX fuel pellets and their dissolution behaviour

International audienceL'abstract porte sur les liens pouvant être établis entre la microstructure (distribution U/Pu mesurée par mcriosonde électronique) de combustibles MOX pour REL et RNR et les résidus issus d'essais de dissolution de ces mêmes combustibles

Modelling of Neodymium Oxalate Precipitation by the Method of Classes

12th International Conference on Chemical and Process Engineering (ICheaP), Milano, ITALY, MAY 19-22, 2015International audienceOxalic precipitation is usually used in nuclear industry to process radioactive wastes and recover actinides from a multi-component solution. To facilitate the development of experimental methods and data acquisitions, actinides are often simulated using lanthanides, thereby gaining experience in harmless conditions. Precipitation reactions being highly sensitive to many operation parameters, modelling appear to be a very effective tool to predict the evolutions of the system under various operating conditions, especially in nuclear environment in which experiments are limited. The aim of this article is to describe the modelling approach of neodymium oxalate precipitation in a continuous MSMPR (Mixed Suspension Mixed Product Removal). Primary nucleation, crystal growth and agglomeration are taken into account in the model. Thermodynamic effects are modelled through activity coefficients which are calculated using the Bromley model. For the nucleation study, experimental runs have been performed in a specific device that allows a micromixing time less than a millisecond. The homogeneous nucleation rate follows the Volmer-Weber equation. The crystal growth rate is first order with respect to the supersaturation and controlled by the surface integration into the crystal lattice according to a screw dislocation mechanism. The agglomeration kernel has been found to be independent of the crystal size. The population balance of the particulate suspension is solved by the method of classes. The particle sizes predicted by the model are in good agreement with the experimental measurements

Effect of the Microstructural Morphology on UO2 Powders

International audienceSeveral UO2 powders with different morphologies were synthetized and characterized. Three different morphologies were synthesized thanks to sol gel process (big heap of about 200μm wide consisting of sintered crystallites) on the one hand, and to oxalic precipitations (one square platelet morphology and one hexagonal stick morphology) on the other hand. Significant differences in dissolution kinetics were observed. Therefore, the morphology of the powders was found to be a key parameter that has to be considered in studies of UO2 dissolution kinetics. The second part of the study consists in dissolving in nitric acid in in the same operating conditions three UO2 powders having different crystallites sizes. It was shown that dissolution kinetics is dependent on the morphology but also on the powder stoichiometry