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

In situ HT-ESEM oxidation of sintered UC fragments revealed the morphological changes occurring during the transformation between UC to UO₂ and UO₂ to U₃O₈ at 723-848 K and in an atmosphere of 10 - 100 Pa O₂. Two main oxidation pathways were revealed. Oxidation at 723 K in atmospheres 25 Pa O₂ showed the transformation from UC to UO_2+x, as confirmed by post mortem HRTEM analysis. This oxidation pathway was comprised of three steps: (i) an induction period, where only surface UC particles oxidised, (ii) a sample area expansion accompanied by crack formation and propagation, (iii) a stabilisation of the total crack length inferring that crack propagation had stopped. Samples oxidised under 50 Pa O₂ at 723 K and at 773-848 K for 10-100 Pa O₂ showed an 'explosive' oxidation pathway: (i) sample area expansion occurred as soon as oxygen was inserted into the chamber and crack propagation and crack length followed an exponential law; (ii) cracks propagated as a network and the oxide layer fragmented, (iii) an 'explosion' occurred causing a popcorn-like transformation, typical for oxidation from UO₂ to U₃O₈. HRTEM characterisation revealed U₃O₈ preferentially grow in the [001] direction. The explosive growth, triggered by ignition of UC, proceeded as a self-propagating high-temperature synthesis reaction, with a propagation speed of 150-500 ± 50 mm/s. (authors)