Oxidation of UC: an in-situ high temperature environmental scanning electron microscopy study

Uranium carbide (UC) has the potential to be used as fuel in Generation IV nuclear reactors thanks to its higher metal atom density and better thermal conductivity when compared to the most commonly used fuel: uranium dioxide (UO2) [1]. Although UC offers improved properties during operation, it needs to be converted into an oxide form after usage as it is reactive and potentially pyrophoric [2] in oxidising environments. Previous oxidation studies on UC, performed over a range of oxygen atmospheres and temperatures, suggest different mechanisms lead to the formation of either a protective or a pulverised non-protective oxide layer [3]. New experimental observations of the oxidation and self-ignition of UC were reported in our previous work [4] involving a combination of state-of-the-art techniques: high temperature environmental scanning electron microscopy (HT-ESEM), high-resolution transmission electron microscopy (HRTEM) combined with an image analysis technique (ImageJ). In situ HT-ESEM oxidation of sintered UC fragments from 723 to 848 K in 10 to 100 Pa oxygen atmosphere revealed the morphological changes to the oxide during the transformations between UC to UO2 and UO2 to U3O8. Oxidation at 723 K in a low O2 atmosphere (≤ 25 Pa O2) produced a compact UO2+x oxide layer, confirmed by post mortem HRTEM analysis. The oxide formed after an induction period and it was accompanied by an exponential followed by logarithmic sample area expansion and crack propagation. Furthermore, samples oxidised at 50 Pa O2 at 723 K and at 773-848 K in an oxygen atmosphere of 10 to 100 Pa O2 showed “explosive” oxidation (see Figure 1). Sample expansion and crack propagation are well described by an exponential law until the “explosion” occurred causing a transformation to a popcorn-like morphology which is typical for oxidation from UO2 to U3O8. HRTEM analysis on the sample powder showed the oxide to be formed of a mixture of U3O7/U3O8 with U3O8 showing preferential growth in the [001] direction. The explosive nature of the oxide is triggered by ignition of UC, which set off this reaction throughout the entire sample with a propagation speed of 150-500 ± 50 µm/s, which shows similarities to a self-propagating high-temperature synthesis reaction. Please click Additional Files below to see the full abstract

On the stoichiometry of zirconium carbide.

The dependencies of the enhanced thermomechanical properties of zirconium carbide (ZrCx) with sample purity and stoichiometry are still not understood due to discrepancies in the literature. Multiple researchers have recently reported a linear relation between the carbon to zirconium atomic ratio (C/Zr) and the lattice parameter, in contrast with a more established relationship that suggests that the lattice parameter value attains a maximum value at a C/Zr ~ 0.83. In this study, the relationship between C/Zr atomic ratio and the lattice parameter is critically assessed: it is found that recent studies reporting the thermophysical properties of ZrCx have unintentionally produced and characterised samples containing zirconium oxycarbide. To avoid such erroneous characterization of ZrCx thermophysical properties in the future, we propose a method for the accurate measurement of the stoichiometry of ZrCx using three independent experimental techniques, namely: elemental analysis, thermogravimetric analysis and nuclear magnetic resonance spectroscopy. Although a large scatter in the results (ΔC/Zr = 0.07) from these different techniques was found when used independently, when combining the techniques together consistent values of x in ZrCx were obtained

Caractérisation structurale d'oxydes mixtes MIV1-xLnIIIxO2-x/2 (M = Ce, Th) préparés par voie oxalique. Etude multiparamétrique de la dissolution et évolution microstructurale.

In the framework of GenIV program development, several physico-chemical properties of some foreseen fuels, including the chemical durability, have to be evaluated. In this aim, a study was undertaken on MIV1-xLnIIIxO2 (M=Ce,Th) model compounds prepared from oxalate precursors. The fluorite-type structure of CeO2 and ThO2 remains stable up to x ≈ 0.4, the substitution of MIV by LnIII occurring simultaneously to the formation of oxygen vacancies. For higher x values, a cubic superstructure is formed as a result of oxygen vacancies ordering. The normalized dissolution rates of such solids were found to be strongly enhanced by the LnIII fraction. On the contrary, the nature of the MIV and LnIII elements did not modify significantly the normalized dissolution rates. The effect of temperature and acid concentration suggested the existence of surface-controlling dissolution reactions. Simultaneously, the microstructural evolution of both powdered and sintered samples revealed some important changes in the reactive surface during dissolution tests. ESEM images allowed observing the existence of preferential dissolution sites located at grains boundaries and around crystalline defects, leading to the formation of corrosion pits. In addition, the formation of gelatinous phases, acting as diffusion barriers (thus slowing down the dissolution process) was also evidenced.Dans le cadre du programme GenIV, les propriétés physico-chimiques d'intérêts des combustibles envisagés, telles que la durabilité chimique, doivent être évaluées. Ainsi, une étude préliminaire a été entreprise sur les composés modèles MIV1-xLnIIIxO2 (M=Ce,Th) preparés à partir de précurseurs oxalate. La structure fluorine caractéristique des oxydes CeO2 et ThO2 demeure stable jusqu'à x ≈ 0,4, la substitution d'ions MIV par LnIII étant accompagnée par la formation de lacunes en oxygène. Pour des valeurs de x plus importantes, une surstructure cubique est formée suite à l'ordonnancement des lacunes en oxygène. Par la suite, les tests de dissolution réalisés en milieu acide ont montré que la vitesse de dissolution normalisée dépend très fortement de la fraction en élément lanthanide incorporé. A l'opposé, la nature des éléments MIV et LnIII constitutifs du solide ne semble que peu modifier la vitesse de dissolution normalisée. Par ailleurs, les effets de paramètres plus " conventionnels " tels que la température ou la concentration en acide ont également été évalués, et ont permis de conclure à une dissolution contrôlée par des réactions de surface. Parallèlement à cette étude, l'évolution microstructurale de composés pulvérulents et frittés a montré d'importantes modifications de la surface réactive durant la dissolution. A partir des observations par MEBE, les joints de grains et les défauts cristallins sont apparus comme des zones préférentielles de dissolution. Par ailleurs, la formation de phases gélatineuses à la surface des solides, agissant comme une barrière de diffusion et ralentissant ainsi la dissolution du matériau a été démontrée

Fabrication and characterization of U1−xAmxO2±δ compounds with high americium contents (x=0.3, 0.4 and 0.5)

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Dilatometric Study of U 1− x Am x O 2±δ Sintering: Determination of Activation Energy

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Hadean isotopic fractionation of xenon retained in deep silicates

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XRD Monitoring of α Self-Irradiation in Uranium–Americium Mixed Oxides

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Experimental and DFT investigation of (Cr,Ti)3AlC2 MAX phases stability

Using a synergistic combination of experimental and computational methods, we shed light on the unusual solubility of (Cr,Ti)3AlC2 MAX phase, showing that it may accommodate Cr only at very low concentrations (<2 at%) or at the exact Cr/(Cr + Ti) ratio of 2/3, even when the ratio of reactants is far from this stoichiometry (1/2 ≤ Cr/(Cr + Ti) ≤ 5/6). In both phases, Cr exclusively occupies the 4f sites, bridging carbide layers with the Al layer. Despite this, the peculiar stability of (Cr2/3Ti1/3)3AlC2 is attributed to the formation of strong, spin-polarized Cr–C bonds, which result in volume reduction and a marked increase in c/a ratio

Application of the UMACS process to highly dense U1−xAmxO2±δ MABB fuel fabrication for the DIAMINO irradiation

International audienc