Modélisation et simulation de la crise d'ébullition dans les REP à l'échelle CFD

Dans un Réacteur à Eau Pressurisée (REP), la chaleur dégagée par le combustible nucléaire est transférée à l’eau du circuit primaire, pressurisée à 150 bars pour éviter son ébullition. Cependant, en situation accidentelle, elle peut entrer en régime d’ébullition nucléée pouvant s’intensifier jusqu’à atteindre la crise d’ébullition. Ce point de transition quasi instantané entre l’ébullition nucléée et l’ébullition en film entraîne la formation d’une couche de vapeur stable sur les crayons combustible, associée à une forte augmentation de leur température pariétale créant un risque de rupture de leur gaine. La prédiction du flux critique (flux de chaleur auquel se produit la crise d’ébullition) représente donc un enjeu de sûreté majeur et est actuellement réalisée à l’aide de corrélations expérimentales spécifiques à une configuration, n’incluant pas de représentation fine de la physique de l’ébullition. Cette thèse s’intéresse à la modélisation de la physique de l’ébullition à l’échelle locale dite « CFD » (Computational Fluid Dynamics), à laquelle il est possible de réaliser des simulations d’écoulements bouillants avec une discrétisation spatiale de l’ordre du millimètre. Le code maison NEPTUNE_CFD, proposant une description eulérienne des écoulements multiphasiques à changement de phase, est l’outil de référence de EDF R&D pour enquêter sur ces problématiques aux échelles locales. Dans un premier temps, des simulations d’écoulements bouillants convectifs en tube vertical sont réalisées avec NEPTUNE_CFD. Des comparaisons avec l’expérience DEBORA (écoulement bouillant de réfrigérant R12 en similitude REP sur plusieurs adimensionnels) ont permis une évaluation du code dans des conditions similaires au cas industriel. Les résultats obtenus sont globalement en accord avec l’expérience, mais présentent des écarts notables sur le diamètre des bulles et la température paroi. Cette dernière est calculée au travers du modèle d’ébullition en paroi de NEPTUNE_CFD dit à « Partition du Flux Pariétal » (Heat Flux Partitioning), où le flux appliqué est découpé entre plusieurs mécanismes de transfert de chaleur (convection, évaporation, conduction instationnaire, etc.). Le cœur des travaux de thèse a alors consisté en la construction d’un nouveau modèle de Partition du Flux, avec objectif une prise en compte plus fine de la phénoménologie de l’ébullition en considérant notamment le glissement des bulles. Une modélisation de la dynamique des bulles en paroi a été développée par une approche mécaniste décrivant les forces appliquées sur la bulle. Les formulations de certaines forces (masse ajoutée, traînée, etc.) ont été réévaluées et permettent une prédiction satisfaisante des diamètres de détachement et des vitesses de glissement à basse et haute pression. Le modèle de Partition du Flux a été complété par une évaluation des nombreuses lois de fermetures requises (temps d’attente, densité de sites de nucléation, etc.) par comparaison avec des mesures expérimentales tirées de la littérature. Le nouveau modèle ainsi développé a ensuite été validé par comparaison avec des mesures de température de paroi et implémenté dans NEPTUNE_CFD. La prédiction du flux critique s’ancre en perspective de ces développements. Des observations expérimentales récentes décrivent la crise d’ébullition à l’aide de paramètres physiques inclus dans le modèle de Partition du Flux. Un critère basé sur la proportion de surface occupée par les bulles a été testé avec l’ancien modèle de NEPTUNE_CFD et semble proposer un comportement qualitativement cohérent. Enfin, on s’intéresse à une configuration de type tube avec des ailettes de mélange similaires à celles présentes en cœur de REP. Les simulations NEPTUNE_CFD montrent des écarts significatifs à l’expérience sur la prédiction du taux de vide à coeur. Des simulations monophasiques montrent une surestimation de la rotation du liquide, pouvant expliquer la trop grande accumulation de vapeur dans le cas bouillant

High-sensitive MIS structures with silicon nanocrystals grown via solid-state dewetting of silicon-on-insulator for solar cell and photodetector applications

This work reports an original method for the fabrication of Metal-Isulator-Semiconductor (MIS) structures with silicon nanocrystals (Si NCs) based active layers embedded in the insulating SiO 2 oxide, for high performance solar cell and photodetector applications. The Si NCs are produced via the in situ solid-state dewetting of ultra-pure amorphous silicon-oninsulator (a-SOI) grown by solid source molecular beam epitaxy (SSMBE). The size and density of Si NCs are precisely tuned by varying the deposited thickness of silicon. The morphological characterization carried out by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) shows that the Si NCs have homogeneous size with welldefined spherical shape and densities up to ~10 12 /cm 2 (inversely proportional to the square of nominal a-Si thickness). The structural investigations by high resolution transmission electron microscopy (HR-TEM) show that the ultra-small Si NCs (with mean diameter ~7 nm) are monocrystalline and free of structural defects. The electrical measurements performed by current versus voltage (I-V) and photocurrent spectroscopies on the Si-NCs based MIS structures prove the efficiency of Si NCs to enhance the electrical conduction in MIS structures and to increase (x10 times) the photocurrent (i.e. at bias voltage V =-1 V) via the photogeneration of additional electron-hole pairs in the MIS structures. These results evidence that the Si NCs obtained by the combination of MBE growth and solid-state dewetting are perfectly suitable for the development of novel high performance optoelectronic devices compatible with the CMOS technology

Universal dephasing in a chiral 1D interacting fermion system

We consider dephasing by interactions in a one-dimensional chiral fermion system (e.g. a Quantum Hall edge state). For finite-range interactions, we calculate the spatial decay of the Green's function at fixed energy, which sets the contrast in a Mach-Zehnder interferometer. Using a physically transparent semiclassical ansatz, we find a power-law decay of the coherence at high energies and zero temperature (T=0), with a universal asymptotic exponent of 1, independent of the interaction strength. We obtain the dephasing rate at T>0 and the fluctuation spectrum acting on an electron.Comment: 5 pages, 3 figures; minor changes, version as published

Shape relaxation of epitaxial mesa for finite-size strain-engineering

Silicon-Germanium (Si$_{1-x}$Ge$_x$) layers are commonly used as stressors in the gate of MOSFET devices. They are expected to introduce a beneficial stress in the drift and channel regions to enhance the electron mobility. When reducing the gate lateral size, one of the major issues is the stress relaxation which results in a significant decrease in the electron mobility. We report a new morphological evolution of a strained epitaxial SiGe nanolayer on a silicon gate (mesa) driven by strain inhomogeneity due to finite-size effects. Unlike the self-induced instability of strained films, this evolution arises here due to the elastic inhomogeneity originating from the free frontiers. We analyze the growth dynamics within the thermodynamic surface diffusion framework accounting for elasticity and capillarity, the former being solved in two dimensions thanks to the Airy formalism. The resulting dynamical equation is solved with a decomposition on eigenmodes, and reveals different developments depending upon the mesa geometric parameters. Mass transfer occurs towards the relaxed areas and creates a beading at the nanolayers free surface with either a W or V shape as a function of time and geometry. The evolution is then controlled by the proportions of the structure as well as its scale.Comment: 10 pages, 5 figure

Magnetic anisotropy in epitaxial Mn5Ge3 films

High crystalline quality Mn 5 Ge 3 films with thicknesses ranging 4–200 nm have been grown on Ge(111) substrates by solid phase epitaxy. The basal hexagonal plane of Mn 5 Ge 3 is in epitaxy with the Ge(111) plane. Magnetic properties of the films have been investigated as a function of the film thickness and the magnetization curves have been analyzed using a theory that includes a description of magnetic domains in uniaxial thin films. The results clearly indicate the existence of a critical thickness below which the magnetic stripe phase disappears. We have determined the value of this thickness to lie between 10 and 25 nm from the analysis of experimental magnetization curves and the theoretical fit of the in-plane remanent magnetization. Although analogies can be drawn between the behavior observed in our system and that of hcp Co, we have shown that the critical thickness is considerably smaller in Mn 5 Ge 3 ; this has the potential to open new fields of applications for Mn 5 Ge 3 thin films in magnetic recording and spintronics

Hyperuniform monocrystalline structures by spinodal solid-state dewetting

Materials featuring anomalous suppression of density fluctuations over large length scales are emerging systems known as disordered hyperuniform. The underlying hidden order renders them appealing for several applications, such as light management and topologically protected electronic states. These applications require scalable fabrication, which is hard to achieve with available top-down approaches. Theoretically, it is known that spinodal decomposition can lead to disordered hyperuniform architectures. Spontaneous formation of stable patterns could thus be a viable path for the bottom-up fabrication of these materials. Here we show that mono-crystalline semiconductor-based structures, in particular Si$_{1-x}$Ge$_{x}$ layers deposited on silicon-on-insulator substrates, can undergo spinodal solid-state dewetting featuring correlated disorder with an effective hyperuniform character. Nano- to micro-metric sized structures targeting specific morphologies and hyperuniform character can be obtained, proving the generality of the approach and paving the way for technological applications of disordered hyperuniform metamaterials. Phase-field simulations explain the underlying non-linear dynamics and the physical origin of the emerging patterns.Comment: 6 pages, 3 figures, supplementary information (7 pages) enclose

Studying Potential Side Channel Leakages on an Embedded Biometric Comparison System

We study in this work the potential side channel leakages of a hardware biometric comparison system that has been designed for fingerprints. An embedded biometric system for comparison aims at comparing a stored biometric data with a freshly acquired one without the need to send the stored biometric data outside the system. Here one may try to retrieve the stored data via side channel, similarly as for embedded cryptographic modules where one may try to exploit side channel for attacking the modules. On one hand, we show that we can find partial information by the means of simple Side Channel Analysis that may help to retrieve the stored fingerprint. On the other hand, we illustrate that reconstructing the fingerprint remains not trivial and we give some simple countermeasures to protect further the comparison algorithm

Enhanced nanoscopy of individual CsPbBr3 perovskite nanocrystals using dielectric sub-micrometric antennas

We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots.We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots

Identification of antifungal compounds from the Root Bark of Cordia anisophylla J.S. Mill.

The dichloromethane extract of the root bark of the Panamanian plant Cordia anisophylla J.S. Mill. (Boraginaceae) presented antifungal activity against a susceptible strain of Candida albicans in a bioautography primary screening. The susceptible strain was used to detect minor active compounds that would not have been detected using a classical approach. In order to identify the antimicrobial compounds, the active extract was fractionated by semi-preparative high-performance liquid chromatography and the fractions were submitted to the antifungal bioassay. This procedure enabled a precise localization of the antifungal compounds directly in the chromatogram of the crude extract and allowed for an efficient, targeted isolation. Four compounds were isolated, one of which is a new natural product. The structures were elucidated using spectroscopic methods. Their antifungal properties were evaluated by determination of the minimum inhibitory quantity and concentration by bioautography and dilution assay against a wild type strain of C. albicans

Population Dynamics of the Critically Endangered\ud Golden Lancehead Pitviper, Bothrops insularis: Stability\ud or Decline?

Little is known about vital rates of snakes generally because of the difficulty in collecting data. Here we used a robust design\ud mark-recapture model to estimate survival, behavioral effects on capture probability, temporary emigration, abundance and\ud test the hypothesis of population decline in the golden lancehead pitviper, Bothrops insularis, an endemic and critically\ud endangered species from southeastern Brazil. We collected data at irregular intervals over ten occasions from 2002 to 2010.\ud Survival was slightly higher in the wet season than in the dry season. Temporal emigration was high, indicating the\ud importance of accounting for this parameter both in the sampling design and modeling. No behavioral effects were\ud detected on capture probability. We detected an average annual population decrease (l= 0.93, CI = 0.47–1.38) during the\ud study period, but estimates included high uncertainty, and caution in interpretation is needed. We discuss the potential\ud effects of the illegal removal of individuals and the implications of the vital rates obtained for the future persistence and\ud conservation of this endemic, endangered species