Combustibles nucléaires

International audienceLe combustible est l'un des éléments essentiels d'un réacteur nucléaire. En son sein ont lieu les réactions nucléaires de fission des atomes lourds, uranium et plutonium. Il est au cœur du réacteur, mais également au cœur du système nucléaire dans son ensemble. Sa conception et ses propriétés influent sur le comportement du réacteur, ses performances et sa sûreté.Même s'il rentre assez faiblement dans le coût du kilowatt-heure produit par les centrales nucléaires actuelles, sa bonne utilisation représente un enjeu économique important. De grands progrès restent à accomplir pour augmenter son temps de séjour en réacteur, lui permettre ainsi de fournir davantage d'énergie, améliorer sa robustesse.Au-delà de l'économie et de la sûreté, des questions aussi stratégiques que l'utilisation du plutonium, la préservation des ressources et la gestion des déchets nucléaires se posent, et de véritables défis technologiques surgissent.Cette monographie résume les connaissances actuelles sur le combustible nucléaire, son comportement en réacteur, ses limites d'utilisation et ses pistes de R&D. Elle illustre également les recherches en cours en présentant quelques résultats marquants obtenus récemment

Polarization measurements analysis II. Best estimators of polarization fraction and angle

With the forthcoming release of high precision polarization measurements, such as from the Planck satellite, it becomes critical to evaluate the performance of estimators for the polarization fraction and angle. These two physical quantities suffer from a well-known bias in the presence of measurement noise, as has been described in part I of this series. In this paper, part II of the series, we explore the extent to which various estimators may correct the bias. Traditional frequentist estimators of the polarization fraction are compared with two recent estimators: one inspired by a Bayesian analysis and a second following an asymptotic method. We investigate the sensitivity of these estimators to the asymmetry of the covariance matrix which may vary over large datasets. We present for the first time a comparison among polarization angle estimators, and evaluate the statistical bias on the angle that appears when the covariance matrix exhibits effective ellipticity. We also address the question of the accuracy of the polarization fraction and angle uncertainty estimators. The methods linked to the credible intervals and to the variance estimates are tested against the robust confidence interval method. From this pool of estimators, we build recipes adapted to different use-cases: build a mask, compute large maps, and deal with low S/N data. More generally, we show that the traditional estimators suffer from discontinuous distributions at low S/N, while the asymptotic and Bayesian methods do not. Attention is given to the shape of the output distribution of the estimators, and is compared with a Gaussian. In this regard, the new asymptotic method presents the best performance, while the Bayesian output distribution is shown to be strongly asymmetric with a sharp cut at low S/N.Finally, we present an optimization of the estimator derived from the Bayesian analysis using adapted priors

Preserved lignin structures in Miocene-aged lignite lithotypes, Bulgaria.

Contents of preserved lignin structures in Miocene-aged lignite lithotypes were determined. Phenol aldehydes, ketones and acids released from lignin by CuO oxidative hydrolysis were separated by capillary zone electrophoresis (CZE). Products of degradation were separated and identified by GC–MS to confirm CZE data and to complement composition data. The following values for the contents of preserved lignin structures in lignite lithotypes were determined (in wt%): xylain, 8.5; humovitrain, 6.7; liptain, 6.4; and humoclarain, 3.3. Some fatty acids, negligible contents of benzene polycarboxylic acids and their hydroxy/methoxy counterparts were also observed using GC–MS

Low temperature reflectivity study of ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates

We report growth of high quality ZnO/Zn0.8Mg0.2O quantum well on M-plane oriented ZnO substrates. The optical properties of these quantum wells are studied by using reflectance spectroscopy. The optical spectra reveal strong in-plane optical anisotropies, as predicted by group theory, and marked reflectance structures, as an evidence of good interface morphologies. Signatures ofc onfined excitons built from the spin-orbit split-off valence band, the analog of exciton C in bulk ZnO are detected in normal incidence reflectivity experiments using a photon polarized along the c axis of the wurtzite lattice. Experiments performed in the context of an orthogonal photon polarization, at 90^{\circ}; of this axis, reveal confined states analogs of A and B bulk excitons. Envelope function calculations which include excitonic interaction nicely account for the experimental report

High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots

We compare the quality factor values of the whispery gallery modes of microdisks incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor (Q) resonant modes on the whole spectrum which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor which reflect the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs based microdisks show very high Q values (Q > 7000) whereas the Q factor is only up to 2000 in microdisks embedding QDs grown on AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation

Organic matter sources and early diagenetic degradation in a tropical peaty marsh (Tritrivakely, Madagascar). Implications for environmental reconstruction during the Sub-Atlantic

Peat samples from a one metre core and living Cyperaceae, collected in Tritrivakely marsh in Madagascar, were studied to determine the organic matter (OM) composition and extent of OM degradation in this core. The study was carried out combining light microscopy observations, bulk analyses, infra-red spectroscopy, hydrolyses of sugars, oxidation of lignin and pyrolyses. In the surface peat, organic matter derived from Cyperaceae undergoes extensive degradation of its basic cell wall components, morphologically revealed by destructuration of plant tissues and their transformation into reddish amorphous organic matter occurring in large amounts all along the core. Two ratios (cinnamic units/lignin and xylose+arabinose/total sugars) were determined as markers of Cyperaceae. It appeared that the vegetation of the marsh remained probably unchanged during the considered accumulation period, i.e. the last 2300 years B.P. Rhamnose, mannose and non-cellulosic glucose probably have a common origin and are mostly derived from bacteria. In contrast, galactose is likely to be a marker of algal source, especially of the diatoms that occur only in the upper part of the core (0-ca. 50 cm). Acid/aldehyde ratios of syringic and vanillic monomers (index of lignin oxidative depolymerisation) and mannose+rhamnose+non-cellulosic glucose/total sugars ratios (reflecting bacterial degradation of hemicelluloses) are positively correlated, and can thus be considered as markers of microbial degradation of the Cyperaceae tissues. The n-alkane/n-alk-1-ene doublets that dominate the pyrolysates of hydrolysed peat samples reflect the contribution of B. braunii algaenan and higher plant suberans, and of condensed lipids mostly derived from higher plants and microalgae. The upper part of the core is characterised by a greater dilution of Cyperaceae-derived compounds by organic matter from microalgae when compared with deeper samples, as recorded by peat bulk features, hydrolysable sugars, lignin oxidation products and pyrolysis products. Two accumulation periods can thus be distinguished in the core: a peaty phase between 2300 years B.P. and ca. 1500 years B.P. (low watertable and strongly limited microalgal growth); a waterlogged marsh, from ca. 1500 years B.P. to the present time, in which a higher water table was longer lasting with a substantial algal production. The environmental variation thus recorded could correspond to a regional climatic change occurring around 1500 years B.P

Museum augmented interface for historical scale models: towards a new way for cultural heritage promotion

International audienceIn this paper, we describe an interactive museum application dedicated to historical scale models. This comes from a joint work between multidisciplinary teams: industrial engineering researchers, historians, museum curators and interactive interface designers. We present here theresult of the project, based on scientific methodology. Results include system architecture, hardware and software, some use cases and user evaluation figures. This paper also underlines some methodology issues that illustrate future possibilities

Sensibilité de la sédimentation organique aux variations climatiques du Tardi-Wuerm et de l'Holocène; le lac du Bouchet (Haute-Loire, France)

Deux carottes de sédiments superficiels (2 à 3 m) prélevées dans le lac du Bouchet (Haute Loire, France) ont été étudiées du point de vue de leur composition minérale et organique. Les corrélations établies à partir des données concernant lamatière organique ainsi que celles de la palynologie, ont permis de replacer les deux premiers mètres de la série sédimentaire dans la logique climato-stratigraphique du passage Tardi-Würm-Holocène. Nous montrons ici que des variations climatiques de faible amplitude comme celles de l'Holocène, engendrent des changements importants dans la nature, la quantité et le mode de préservation des constituants organiques sédimentés

Laser emission with excitonic gain in a ZnO planar microcavity

The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from T=80 K to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process