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International audience; One of the hypothetical accidents studied in the field of the safety studies of Pressurized light Water Reactors (PWR) is the Loss-Of-Coolant Accident (LOCA). In this scenario, zirconium alloy fuel claddings could undergo a high level of oxidation at high temperature in a steam environment. Cladding tubes constitute the first confinement barrier of radioelements and then it is essential that they keep a certain level of ductility after quenching to ensure their integrity. These properties are directly related to the growth kinetics of both the oxide and the sub-oxide αZr(O) layers and especially to the oxygen diffusion profiles in the residual prior-β layer after HT oxidation and final water quenching. This study was focused on the influence of a pre-annealing thermal treatment performed on first generation of 6-8 µm thick Cr-coated zirconium based specimens and on their further HT oxidation behavior. The effect of pre-annealing temperature on the structure and morphology of the Chromium coating microstructure was investigated using scanning electron microscopy. During the annealing treatment, the chromium coating experienced recrystallisation and the initial columnar grains morphology became more equiaxed. After oxidation at HT, the weight gains of pre-annealed Cr-coated specimen were generally lower than the non-annealed ones. Glow discharge mass spectrometry, Electron Probe Micro Analysis and Raman spectroscopy measurements have confirmed that the thickness of ZrO2 and αZr(O) phases were significantly reduced for some of the pre-annealed samples, indicating that the grain size and morphological evolutions of the chromium coating upon the pre-annealing thermal treatment may influence its further HT oxidation behavior.

International audience; Low-alloy ferritic steel (16MND5) is used for pressure vessels in French nuclear reactors, where radiationinduced defects are continuously nucleated during their operational lifetime. More specifically, such defects have a major influence on the collective behavior of dislocations, leading to a significant hardening and embrittlement. A multiscale modeling approach based on a combination of molecular dynamics and dislocation dynamics simulations is therefore adopted. Interactions of edge dislocations with radiation-induced defects have been widely studied compared to those of screw dislocations. The later are nevertheless of great interest as their specific mobility has a significant impact on the mechanical properties of steels at low and moderate temperatures. Furthermore, prior molecular dynamics(MD) studies[1, 2] have shown that their interactions with irradiation loopslead to significant hardening. In this study, a systematic investigation is made of the interactions between screw dislocations with [100] and [111] loops using a three-dimensional nodal dislocation dynamics (DD) code NUMODIS. The same configurations that were previously simulated with MD are successfully reproduced. This allows for a direct comparison between atomic scale and DD simulations to validate our multiscale strategy. Ultimately, the modeling of the collective behavior of dislocations properties controlling irradiation strengthening will be undertaken in order to identify and incorporate the relevant parameters in a crystal plasticity simulation.

International audience; Du fait de leurs bonnes proprietes mecaniques et de leur faible activation neutronique, les alliages de titane sont susceptibles d'etre utilises comme materiaux de structure de composants du circuit primaire des Reacteurs nucleaires a Eau Pressurisee (REP). Les objectifs de la presente etude sont de determiner les mecanismes de corrosion du titane dans les conditions du milieu primaire des REP, puis d'en mesurer et d'en modeliser les cinetiques. Pour cela, des echantillons de T40 (titane commercialement pur monophase alpha) ont ete exposes au milieu primaire en boucle de corrosion pour des durees allant de 24 h a 5 mois a 300 DC et de 100 h a 2,5 mois a 350 DC.Les mecanismes de corrosion ont ete etablis a l'aide de diverses techniques d'analyses. Les observations en microscopie electronique a balayage et en transmission ont mis en evidence d'une part l'existence d'une fine couche dense de TiO2 rutile dont l'epaisseur reste constante avec la duree d'exposition et d'autre part l'augmentation de la tortuosite de l'interface oxyde/milieu avec le temps. Cela demontre l'existence d'un phenomene de dissolution de l'oxyde dans le milieu et l'atteinte du regime stationnaire pour lequel la vitesse de dissolution et la vitesse d'oxydation sont egales. La prise de masse des echantillons et les observations en microscopie electronique indiquent qu'une proportion importante des oxydes dissous reprecipite sous la forme de cristallites a la surface des echantillons. Les analyses par spectrometrie Raman, diffraction des rayons X et microscopie en transmission ont montre que ces cristallites d'oxydes de surface sont constituees de TiO2 anatase et de FeTiO3 ilmenite.Un modele paralineaire est propose afin de quantifier la cinetique de corrosion resultante de trois phenomenes concomitants l'oxydation du metal, la dissolution de l'oxyde dans le milieu et la precipitation d'une partie de l'oxyde dissous a la surface du materiau. Ce modele cinetique sera enfin compare aux cinetiques experimentales obtenues grace aux prises de masse, aux observations par microscopie electronique a balayage et aux analyses par spectroscopie a decharge luminescente.

International audience; In surface microanalysis with sampling by laser ablation (LIBS, LA-ICP-OES or LA-ICP-MS methods), spatial resolution is determined by laser beam diffraction limits (of the order of a laser wavelength) and thermal diffusion of a deposited heating energy during laser pulse duration (proportional to the square root of the pulse duration and matter diffusivity). Being limited by these features of a laser beam and heating energy, the best LIBS spatial resolution (the crater diameter) of 1 µm was obtained by ablation with the laser pulses of 266 nm wavelength and 4 ns duration [1-2]. One of the ways to improve spatial resolution of microanalysis may be suggested as the use of laser pulses of a lower wavelength ( 266 nm) and shorter pulse durations (ps and fs). Other ways to improve spatial resolution of surface analysis is laser ablation with a highly localized electromagnetic field produced by a tip near-field enhancement. In our studies, a tip of an atomic force microscope (AFM) was illuminated by the laser pulses of 266 nm wavelength and 4 ns duration, thus producing the craters of 100 nm diameters and of some nanometers of depth with Si-, Au- and Ta-samples [3]. The experiments were followed by extensive multi-parametric theoretical studies to analyze the effect of both matter properties (absorption, thermal conductivity and capacity) and electromagnetic field parameters (spatial distribution and pulse duration) on the resulted temperature field distribution T(r, t). The simulations were made with a home-developed “3D+t” heating model [4]. The simulation results [5] with particular features of solid sample heating by highly spatially localized laser field (10-100 nm) will be presented and discussed.

International audience; Among the numerous laser applications, laser surface melting by using a pulsed-laser is an innovative technology in the field of surface treatments. This technique presents many advantages. It only modifies the surface properties by keeping the mechanical properties of the bulk. It requires neither addition of other compounds nor contact, so it is quite economical and it does not pollute the material. It allows the treatment of complex shapes into closed spaces with difficult access. The laser can work in autonomy that present an interest in the fields of nuclear decontamination.This treatment consists in focusing a nanopulsed laser beam on the surface of the material, leading to the rather immediate melting of the surface through a micron depth, immediately followed by an ultra-fast solidification occurring with cooling rate up to 1010 K/s.By using different techniques of analysis, we showed that the combination of these processes leads to various modifications of surface properties. By combining with dexterity the different laser parameters, it is possible to functionalize the surface or to improve the native properties.Glow discharge optical emission spectrometry (GDOES), XPS and TEM were used to establish the segregation of chemical elements and the growth of a new oxide layer with new properties.XRD with grazing incidence was employed to identify the change of crystallographic structure, SEM was performed to promote the diminution of the surface defects and the microstructure.Applications in fields of nuclear will be presented, especially in terms of pitting corrosion of stainless steel used in secondary circuit, and protection against the nickel release of heat exchanger tubes in the primary circuit coolant.

International audience; A method of near-field (NF) laser ablation for sampling of solids with a nm resolution was developed and applied. The method is based on the coupling of nanosecond or femtosecond laser pulses with an Atomic Force Microscope (AFM). The technique uses a tip enhancement effect obtained by interaction of laser radiation with a tip of the AFM maintained a few nm above a sample surface. In our experiments, craters of about 100 nm in diameter and a few nm in depth were obtained with a single laser pulse on gold and silicon samples. The elemental or isotope content of the ablated matter may be analyzed by a mass spectrometer with inductively coupled plasma source (ICP-MS). An electrostatic model of the NF created by an ellipsoidal tip with a coating was used in NF heating and ablation calculations. It allows one to take into account the effect of possible plasmonic resonance (both in the tip and the surface under study) on the NF enhancement factor. A two-temperature model of electron and lattice heating was used for temperature calculations for both heating and ablation regimes. A multi-parametric study was made to understand the effect of different experimental parameters (laser fluence, tip-to-sample distance, sample and tip nature) on NF laser ablation efficiency, crater dimensions and amount of ablated material. The developed combined model of the NF heating and ablation was used to test calculations and to study different interaction features. Numerical simulations of the NF localized heating with a 3-D code have offered a good explanation of nm crater diameters obtained in the experiments.

International audience; Among the numerous laser applications, laser surface melting by using a pulsed-laser is an innovative technology in the field of surface treatments. This technique presents many advantages. It only modifies the surface properties by keeping the mechanical properties of the bulk. It requires neither addition of other compounds nor contact, so it is quite economical and it does not pollute the material. It allows the treatment of complex shapes into closed spaces with difficult access. The laser can work in autonomy that present an interest in the fields of nuclear decontamination.This treatment consists in focusing a nanopulsed laser beam on the surface of the material, leading to the rather immediate melting of the surface through a micron depth, immediately followed by an ultra-fast solidification occurring with cooling rate up to 1010 K/s.By using different techniques of analysis, we showed that the combination of these processes leads to various modifications of surface properties. Glow discharge optical emission spectrometry (GDOES) and transmission electronic microscopy (TEM) were used to establish the segregation of chemical elements and the growth of a new oxide layer with new properties.X-Ray diffraction (XRD) with grazing incidence was employed to identify the change of crystallographic structure, scanning electron microscopy (SEM) was performed to promote the diminution of the surface defects and the roughness was also characterized.Applications in fields of nuclear, corrosion, and decontamination will be presented. Those results showed that the new surface properties strongly depend of the laser parameters.

National audience; Les alliages de titane pourraient potentiellement etre utilises comme materiaux de structure de composants du circuit primaire des Reacteurs a Eau Pressurisee (REP) du fait de leur faible activation neutronique et de leur bonne resistance a la corrosion. Neanmoins, ils sont connus pour etre enclins a la prise d'hydrogene, phenomene susceptible d'alterer leurs proprietes mecaniques. L'objectif de cette etude est donc de determiner la cinetique et le mecanisme de prise d'hydrogene dans les conditions du milieu primaire des REP ainsi que d'evaluer l'influence de celle-ci sur le comportement mecanique des alliages de titane. Pour cela, trois alliages de titane representatif des differentes classes industrielles le T40 (phase alpha), le TA6V (4 % de phase beta) et le Ti10-2-3 (40 % de phase beta) ont ete expose au milieu primaire en autoclave et en boucle de corrosion. Des analyses de dosage par fusion totale et de Spectroscopie a Decharge Luminescente (SDL) ont ensuite ete realisees afin d'estimer les quantites d'hydrogene absorbees et de localiser ce dernier au sein des materiaux d'interet. Par ailleurs, ces materiaux ont ete charges en hydrogene par voie electrochimique afin d'obtenir des concentrations en hydrogene equivalentes a celles qui seraient obtenues au cours de plusieurs annees d'oxydation. Des analyses quantitatives par spectrometrie de desorption thermique ont aussi ete realisees sur des echantillons charges en deuterium afin de caracteriser les phenomenes de piegeage de l'hydrogene. Le comportement mecanique des echantillons exposes au milieu primaire ainsi que des echantillons charges par voie cathodique a ete evalue par des essais de traction et de fluage en traction. Les premiers resultats de cette etude seront presentes et discutes.

International audience; Because of their good resistance to corrosion in acidic media, austenitic stainless steels are largely used for equipment of the spent nuclear fuel reprocessing plants. Still, they might be exposed to an intergranular type of corrosion. The intergranular corrosion appears in the transpassive domain of the steel, for example when the chemical environment becomes sufficiently oxidizing. However, when few wt.% of silicon are added to a 18Cr-10Ni stainless steel, the alloy loses its sensitivity to intergranular attack. This work compares the transpassive corrosion behavior of two similar stainless steel that are differentiated by the addition of silicon in their composition. Using the in-situ atomic emission spectroelectrochemistry (AESEC), chronoamperometries were performed in the transpassive domain of each steel. During the potentiostatic transpassive dissolution of each sample, an inductive coupled plasma atomic emission spectrometer connected to a three electrode flow cell enabled to quantify each element's dissolution rate. Then, scanning electron micrographs were taken to evaluate the surface's morphology after the experiment. As expected, the steel without silicon displayed both a selective dissolution behavior according to AESEC measurements and a locally corroded surface, very sensitive to the flow rate, according to SEM micrographs. On the other hand, the silicon-rich stainless steel displayed a non-selective dissolution behavior and a homogeneous surface morphology. Transmission electron microscopy was then used in addition to this work, to understand better the dissolution mechanisms in which the silicon is involved from its metallurgical characteristics in the bulk and in the passive layer. Comparing the total current measured and the elemental dissolution rates, the valence of each element dissolving was determined. In this work, in-situ and ex-situ techniques lead to the suggestion of a transpassive dissolution mechanism for each stainless steel as a function of silicon enrichment.

International audience

International audience; In the framework of research on Generation IV nuclear power plant, French research is focused on Sodium Fast Reactor. In the proposed design, heat exchange between the reactor core and the secondary circuit is performed via the Energy Conversion System (ECS). This specific ECS design involves that sodium (from the core) heat is transferred to gaseous nitrogen (from the secondary circuit) through 2 mm thickness wall channels. The considered material for this application is the 316L stainless steel. However this stainless steel grade has not been studied in nitrogen environment at working conditions (180 bar and 515DC) and few data are available in literature to assess nitriding risk.

International audience; To satisfy the growing demands of modern science and industry, laser ablation methods are under continuous development. The development of laser ablation methods has significantly expanded the scope of their applications for modern laser-assisted micro and nanotechnologies. In particular, in our studies on near-field laser ablation, the possibility to perform laser ablation and surface processing with a nanometric resolution was demonstrated with laser field enhancement near a tip of an atomic-force microscope. Laser ablation, in our case, may be regarded as a result of the local near-field heating of a sample surface. Laser ablation efficiency and accuracy are affected by both the sample properties (thermal diffusivity, absorption coefficient, etc.) and the near-field parameters (spatial distribution, wavelength, laser pulse duration, tips nature and dimension, etc.). The modeling of the temperature distribution resulting from the near-field heating of the sample surface may be seen as an appropriate procedure to analyze these complex multi parametric effects and corresponding interrelations. The paper presents the results on the modeling for the near-field heating of metal or semiconductor surfaces (gold, tantalum and silicon wafer) along with those obtained experimentally with nanosecond laser pulses (4 ns, 266 nm, 10 Hz, linear p-polarization) and an atomic-force microscope in air, under normal conditions. The properties of nanometer scale craters are in good correlation with the model predictions. The possible improvements of the near-field ablation with a femtosecond laser will be discussed.

International audience

International audience; A near field laser ablation inductively coupled plasma mass spectroscopic method (NF-LA-ICPMS) was developed for chemical analysis at sub-micrometric scale. This analytical technique combines a nanosecond laser, an Atomic Force Microscope (AFM), and a high resolution ICP-MS. In order to improve the spatial resolution of the laser ablation process, the near field enhancement effect was applied by illuminating the apex of a conductive sharp tip of the AFM by a laser beam. The laser energy was kept below the sample ablation threshold, and a distance of a few nanometers (5 to 30 nm) was maintained between the tip and the surface sample. The interaction between the illuminated tip and the sample surface enhances locally the incident laser irradiance and leads to the ablation process.

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International audience; Austenitic stainless steels (SS) with silicon additions are used for some equipment containing nitric acid in the spent nuclear fuel reprocessing plant. This work aimed at studying the corrosion behavior of this SS in such acid media. Literature expects the passive layer's evolution to be related to the corrosion potential shifts, even within the passive state. Based on electrical transient currents analysis and X-ray photoelectron spectroscopy, it is shown that its composition is deeply modified and the reversibility of these modifications is unknown. To study these modifications, two kinds of electrochemical experiments were performed linear polarizations from active to transpassive domains to study the oxidation/dissolution behavior as a function of potential and alternative polarizations between active and passive states to study the passive layer formation/dissolution kinetics. In both cases, the measurement was followed using in-situ Atomic Emission Spectro ElectroChemistry (inductive coupled plasma atomic emission spectroscope real time connected to a potentiostat) and chemical surface evolution was characterized with ex-situ X-ray photoelectron spectroscopy. An accumulation on surface (resulting from a selective dissolution) of several elements was found as a function of potential. In the active domain, metallic copper accumulates on surface and dissolves intensely at the end of the active domain, which affects the cathodic reaction kinetics. In the passive domain, surface accumulation of chromium was found to be reversible when polarized again in the active domain. This phenomenon directly controls the SS global dissolution rate. Eventually, accumulation of silicon at the surface was found in both active and passive domains. Hence, this enrichment is more likely to be localized at the surface of the sample, hypothesis which is investigated by XPS.

International audience; Zirconium is used as material for various applications in spent nuclear fuel reprocessing plants involving concentrated nitric acid medium under highly corrosive oxidizing conditions. In such conditions, zirconium can show two different oxidation regimes as a function of potential. At high potentials, the oxidation current is relatively important (few $\mu$A/cm$^2$) and constant with time. A poorly protective black oxide layer (ZrO$_2$) is formed on the surface, which has a thickness of few micrometers. At low potentials, the oxidation current decreases with time and progressively tends to low values (few nA/cm$^2$). This protective effect has been attributed to the formation/growth of a nanometric oxide layer (ZrO$_2$) on the surface. The thickness evolution of this layer has been simultaneously estimated as a function of time by in-situ electrochemical methods (Electrochemical Impedance Spectroscopy (EIS) and coulometry) and by ex-situ X-ray Photoelectron Spectroscopy analyses (XPS). These experimental results have been successfully compared with an anodic oxide layer growth model based on the high field regime. The addition of small amounts of fluorine in nitric acid (a few tenths of millimoles per liter) modifies notably the corrosion processes. Three phases are observed in the corrosion mechanism:(1) a first incubation phase where the corrosion rate is negligible, (2) a second phase with a sharp increase of the corrosion rate (the SEM observations show a process of nucleation / growth /coalescence of pits) and (3) a third phase with a progressive decrease of the corrosion rate (corresponding to a generalized corrosion mechanism by an oxidation/complexation process limited by the supply of the reactive species)

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International audience; Ni-Cr-W alloys are claimed to present high creep strength, a good hot workability together with improved oxidation resistance. Recently developed Haynes 230 could be a promising candidate material for the high temperature structures in Gas Cooled Reactors (GCR). However, corrosion resistance of Haynes 230 is unknown in the reactor specific environment namely impure He up to 850-950DC. Generally speaking the impurities in the cooling helium react with most Cr-containing alloys. They can produce surface scale formation, bulk carburisation and-or decarburization, depending on the gas chemistry, the alloy composition and the temperature. The changes in the bulk carbon content and the associated structural transformations can notably degrade the material mechanical properties and must definitely be precluded. Corrosion properties of Haynes 230 were investigated in a purposely-designed facility under different helium media at about 950DC. Short and longer experiments were carried out in simulated GCR coolant in order to assess the corrosion variability to test conditions. More specifically, the oxide scale formation and a deleterious reaction destructing the protective oxide layer were studied. Main factors impacting on this previous reaction, such as the carbon monoxide partial pressure in the gas atmosphere and the activity of the reactive elements in the alloy were investigated. The reaction mechanism is discussed in the light of published models.