September 17-20, 2017
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) . 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  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 .
New experimental observations of the oxidation and self-ignition of UC were reported in our previous work  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  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.
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Claudia Gasparrini, Michael J.D. Rushton, William E. Lee, Renaud Podor, Denis Horlait, and Olivier Fiquet, "Oxidation of UC: an in-situ high temperature environmental scanning electron microscopy study" in "Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications IV", Jon Binner, The University of Birmingham, Edgbaston, United Kingdom Bill Lee, Imperial College, London, United Kingdom Eds, ECI Symposium Series, (2017). https://dc.engconfintl.org/uhtc_iv/21