[en] Mixed plutonium and uranium monocarbide (UPuC) is considered as a possible fuel material for future nuclear gas fast reactors. Its safe handling is currently a major concern, because inflammation of this material under the shape of fine powders is easy and highly exothermic (pyrophoricity) even under ambient temperature and partial pressure of oxygen inferior to 0.2 bar. CEA Marcoule is implied in both experimental and numerical studies on the UC powder oxidation exothermic reaction. Experimental tests consist in determining the influence of various parameters (gas composition, heating ramp, specific surface of powders) on the sample inflammation temperature. Two kinds of analytical apparatus are used: The differential thermal analysis (DTA) and the differential scanning calorimetry (DSC) coupled to the thermo gravimetric analysis (TGA). These apparatus are also linked to a gas mass spectrometer to follow the composition of combustion chamber gases. Results obtained with small quantities revealed that UC powder is highly reactive in air in the temperature range of 150-250 degrees C and showed a strong dependence between powder height in crucibles and inflammation temperature. (authors)

[en] Niobium nitride (NbN) is widely used in high-frequency superconducting electronics circuits because it has one of the highest superconducting transition temperatures ($T_{\mathrm{c}}\sim <!--\; ???\; -->16.5\mathrm{K}$) and largest gap among conventional superconductors. In its thin-film form, the T _c of NbN is very sensitive to growth conditions and it still remains a challenge to grow NbN thin films (below 50 nm) with high T _c. Here, we report on the superconducting properties of NbN thin films grown by high-temperature chemical vapor deposition (HTCVD). Transport measurements reveal significantly lower disorder than previously reported, characterized by a Ioffe–Regel parameter ($k_{\mathrm{F}}\mathrm{\ell <!--\; ???\; -->}$) ∼ 12. Accordingly we observe $T_{\mathrm{c}}\sim <!--\; ???\; -->17.06\mathrm{K}$ (point of 50% of normal state resistance), the highest value reported so far for films of thickness 50 nm or less, indicating that HTCVD could be particularly useful for growing high quality NbN thin films. (paper)

[en] In this paper a thermodynamic and experimental study in-situ chlorination of tungsten is presented. When gaseous tungsten chlorides are used as metal precursors for tungsten and tungsten silicide CVD, their direct synthesis provide a reproducible volatile chloride source. The conditions leading to the best production of volatile tungsten chlorides have been optimize by means of thermodynamic calculations. The influence of the different production parameters (temperature, nature of reactant gas, etc.) have been determined. During some chlorination experiments, volatile chlorides have been detected in-situ by a quadrupole mass spectrometer. Good agreement between theoretical and experimental results is obtained

[en] The origin of threading dislocations (TDs) in nitride films is not completely understood but it is well established that they degrade the film properties. This work investigates the assumption that they arise from the interface between the film and sapphire substrate owing to small in-plane rotations between nitride domains. Bollmann’s formalism is first used to determine the characteristics of dislocations at the nitride film/sapphire interface that compensate both for the parametric misfit and a small in-plane rotation of the film as frequently observed. It is shown that the dislocation density and line direction depend on the rotation angle. When islands grow and coalesce in the nucleation layer, some interfacial dislocations orientate along [0001] in the boundaries between domains and transform to so-called TDs. The amount of TDs lying in the boundaries between nitride domains is calculated as a function of the rotation angle. Estimations of TD density in the nucleation layer are deduced for a range of domain sizes and compared with experimental values of the literature.

[en] Mixed (U, Pu) carbide, constituted by means of 80% of uranium monocarbide (UC), is considered as a possible fuel material for future gas fast reactors or sodium fast reactor. However, UC undergoes a strong exothermic reaction with air and fine powders of UC are pyrophoric. Thus, it is necessary to understand this high reactivity in order to determine safe handling conditions for the production and reprocessing of carbide fuels. UC powder was obtained by arc melting and milling. The reactivity of uranium carbide was studied in oxidizing atmosphere and different experimental devices were used to determine ignition temperatures. The phases formed at the various observed stages of the oxidation process were determined by post-mortem X ray diffraction analysis. Studies were first performed using small quantities of UC powder (around 50 mg) in Differential Thermal Analysis / Thermogravimetric Analysis (DTA/TGA) and Differential Scanning Calorimetry (DSC). Experiments were realized using different parameters, such as heating rate and gas flow rate and composition, to determine their influence on pyro-phoricity. Results obtained with small quantities (tens of milligrams) revealed that UC powder is highly reactive in air in the range 200- 250 deg. C. Studies were also performed in the 'Pyro' test facility multi-function furnace allowing CCD camera recording, during heating and ignition, through view-ports. Lower ignition temperatures, around 100 deg. C, were obtained using around 1 g UC powder samples. Results are discussed and analysed with theory of burning curve ignition and numerical simulations. Simulations aim to understand the influence of the different parameters on pyro-phoricity. Small scale simulations (on a spherical grain) confirm the influence of UC grains size, heat rate and gas composition on powder ignition temperature with small quantities. The issue is now to understand the influence of grain pile form factor and volume on the pyro-phoricity of UC. Large scale simulations, including mass and heat transport in the UC powder environment and into the pile are now under construction. Ignition is also analysed thermodynamically along isothermal sections of the U C O ternary phase diagrams. Assumptions are proposed concerning the overall oxidation and ignition paths. It appears that the chemical reaction of oxygen with UC grains forms several phases that progress into the grain during the ignition. This phenomenon is going to be included in our simulations. (authors)

[en] The reactivity of the uranium monocarbide and mono-nitride has been studied under air and argon or oxygenated nitrogen in order to determine their inflammability temperature and to search safe experimental conditions for the manufacture and reprocessing of these compounds. These compounds will certainly be used as fuels in the future nuclear reactors. During their treatment, they are pyrophoric and can become strongly overheated when they are in the state of finely separated powders. Experimental techniques as the TGA, the DTA and the DSC have revealed a strong reactivity under air of the UC powder at 200 C. For the UN powder, the inflammability occurs above 300 C. The influence of the oxygen amount of the scanning gas and the velocity of the temperature increase have been studied. A study of the ternary diagram U-C-O has been carried out and a mechanism based on the progressive or sudden rupture of the UO₂ formed layer is proposed. (O.M.)

[en] Plasma etching of HfO₂ at an elevated temperature is investigated in chlorine-based plasmas. Thermodynamic studies are performed in order to determine the most appropriate plasma chemistry. The theoretical calculations show that chlorocarbon gas chemistries (such as CCl₄ or Cl₂-CO) can result in the chemical etching of HfO₂ in the 425-625 K temperature range by forming volatile effluents such as HfCl₄ and CO₂. The etching of HfO₂ is first studied on blanket wafers in a high density Cl₂-CO plasma under low ion energy bombardment conditions (no bias power). Etch rates are presented and discussed with respect to the plasma parameters. The evolution of the etch rate as function of temperature follows an Arrhenius law indicating that the etching comes from chemical reactions. The etch rate of HfO₂ is about 110 A /min at a temperature of 525 K with a selectivity towards SiO₂ of 15. x-ray photoelectron spectroscopy analyses (XPS) reveal that neither carbon nor chlorine is detected on the HfO₂ surface, whereas a chlorine-rich carbon layer is formed on top of the SiO₂ surface leading to the selectivity between HfO₂ and SiO₂. A drift of the HfO₂ etch process is observed according to the chamber walls conditioning due to chlorine-rich carbon coatings formed on the chamber walls in a Cl₂-CO plasma. To get a very reproducible HfO₂ etch process, the best conditioning strategy consists in cleaning the chamber walls with an O₂ plasma between each wafer. The etching of HfO₂ is also performed on patterned wafers using a conventional polysilicon gate. The first result show a slight HfO₂ foot at the bottom of the gate and the presence of hafnium oxide-based residues in the active areas

[en] Mixed plutonium and uranium carbide (UPuC) is considered as a possible fuel material for future nuclear reactors. However, UPuC is pyrophoric and fine powders of UPuC are subject to temperature increase due to oxidation with air and possible ignition during conditioning and handling. In a first approach and to allow easier experimental conditions, this study was undertaken on uranium monocarbide (UC) with the aim to determine safe handling conditions for the production and reprocessing of uranium carbide fuels. The reactivity of uranium monocarbide in oxidizing atmosphere was studied in order to analyze the ignition process. Experimental thermogravimetric analysis (TGA) and differential thermal analysis (DTA) revealed that UC powder obtained by arc melting and milling is highly reactive in air at about 200 deg. C. The phases formed at the various observed stages of the oxidation process were analyzed by X-ray diffraction. At the same time, ignition was analyzed thermodynamically along isothermal sections of the U-C-O ternary diagram and the pressure of the gas produced by the UC + O₂ reaction was calculated. Two possible oxidation schemes were identified on the U-C-O phase diagram and assumptions are proposed concerning the overall oxidation and ignition paths. It is particularly important to understand the mechanisms involved since temperatures as high as 2500 deg. C could be reached, leading to CO(g) production and possibly to a blast effect.

[en] In case of a severe accident in a nuclear power plant, the behaviour of iodine is one of the major issues because of its radiotoxicity. Special attention is paid to the gaseous species as a result of their mobility and so the ease with which they can escape into the environment. During the Phebus-FP tests, a significant fraction of gaseous iodine was observed in the containment that is not well accounted by the simulations. Possible explanations are kinetic limitations or the presence of caesium sinks such as molybdenum in the reactor coolant system that would limit the formation of stable condensed forms mainly CsI. In order to get additional experimental data, a test facility (GAEC test facility) was developed with the aim to study the chemical reactions between iodine, caesium and another element in conditions representative of the primary circuit. This paper presents the first results obtained with GAEC test facility during the vaporization of CsI and MoO₃ under steam. It was clearly observed that molybdenum promotes the fraction of gaseous iodine at 150 C. degrees (temperature of the cold leg). Molecular iodine I₂ was shown to be the predominant iodine gaseous species and caesium poly-molybdates under condensed form were identified by Raman microspectrometry. This last result consolidates the idea that molybdenum acts as a caesium sink. A simple approach is proposed using Gemini2 code and a specific database to reproduce the formation of the caesium molybdates. Simulations performed with the new kinetic model implemented in the SOPHAEROS code gives promising results concerning the fraction of gaseous iodine. (authors)

[en] Uranium monocarbide (UC) powders are known to be easily oxidised by gas mixtures containing oxygen. In this study, the oxidation of UC micron powders was followed by isothermal thermogravimetry at temperatures ranging from 100 °C to 230 °C in two different gas mixtures: synthetic air and 97%N₂ + 3%O₂. X-ray diffraction tests conducted on powders after their oxidation showed that small crystallites of UO₂ oxide are formed. Furthermore, an analysis of mass gain showed that the carbon initially linked with the uranium is not oxidised but retained in oxide layers. Additionally, this kinetic study revealed that the rate-limiting step mechanism governing the oxidation of UC powders is a diffusive process that follows the Arrhenius law regarding temperature. Finally, it was discovered that cracks occur in grains once a given fractional conversion has been reached, inducing a major increase in the volume of grains.