Résumé
$In\ situ$ HT-ESEM oxidation of sintered UC fragments revealed the morphological changes occurring during the transformation between UC to UO$_2$ and UO$_2$ to U$_3$O$_8$ at 723 to 848 K and in an atmosphere of 10 to 100 Pa O$_2$. Two main oxidation pathways were revealed. Oxidation at 723 K in atmospheres ≤ 25 Pa O$_2$ 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 O$_2$ at 723 K and at 773-848 K for 10 to 100 Pa O$_2$ 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 UO$_2$ to U$_3$O$_8$. HRTEM characterisation revealed U$_3$O$_8$ 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 $\mu$m/s.