The oxidative corrosion of carbide inclusions at the surface of uranium metal during exposure to water vapour

► High resolution imagery (FIB, SEM and SIMS) of carbide inclusions in uranium metal. ► Real time images following the reaction of the carbide inclusions with water vapour. ► Shown preferential consumption of carbide over that of the bulk metal. ► Quantity of impurities in the metal therefore seriously influence reaction rate. ► Metal purity must be considered when storing uranium in air or moist conditions. The reaction between uranium and water vapour has been well investigated, however discrepancies exist between the described kinetic laws, pressure dependence of the reaction rate constant and activation energies. Here this problem is looked at by examining the influence of impurities in the form of carbide inclusions on the reaction. Samples of uranium containing 600 ppm carbon were analysed during and after exposure to water vapour at 19 mbar pressure, in an environmental scanning electron microscope (ESEM) system. After water exposure, samples were analysed using secondary ion mass spectrometry (SIMS), focused ion beam (FIB) imaging and sectioning and transmission electron microscopy (TEM) with X-ray diffraction (micro-XRD). The results of the current study indicate that carbide particles on the surface of uranium readily react with water vapour to form voluminous UO 3· xH 2O growths at rates significantly faster than that of the metal. The observation may also have implications for previous experimental studies of uranium–water interactions, where the presence of differing levels of undetected carbide may partly account for the discrepancies observed between datasets.