Oxidation of uranium carbide (UC) was investigated because it is used as a preliminary treatment prior to storage and permanent disposal of carbide nuclear fuels. Working with UC present some challenges, mainly related to its radioactivity and pyrophoricity, therefore initial studies were conducted on zirconium carbide (ZrC) which is isostructural and exhibits similar chemistry to UC. High temperature environmental scanning electron microscopy (HT-ESEM) was used to examine in situ the oxidation of both ZrC and UC. Oxide products were subsequently analysed using macro to nano characterisation techniques, such as focused ion beam secondary ions mass spectroscopy (FIB-SIMS) and high resolution transmission electron microscopy (HRTEM).
Oxidation of ZrC was studied from 1073 to 1373 K in air and at 1073 K in a 200 Pa oxygen atmosphere. UC oxidation was studied from 723 to 1173 K at different oxygen atmospheres (2–100 Pa) and from 873 to 1173 K in air.
A key result was the improved understanding of the role of cracking in the oxidation mechanism of both carbides. Cyclic cracking parallel to the carbide/oxide interface and crack propagation at corners was found to be responsible for the Maltese cross shape of the oxide in ZrC. The oxidation mechanism of ZrC was governed by oxygen diffusion through a layer of constant thickness formed by the cyclic debonding of the interface after the oxide layer reached approximately 20 µm at 1073 K. The interface was an approximately 2 µm thick intermediate layer comprising zirconia nanocrystals (≤5 nm) in an amorphous carbon matrix. Crack length stabilisation was characteristic of UC oxidation to UO2+x while an exponential increase of crack length triggered an explosive transformation producing U3O8 in samples oxidised from 723 to 848 K in 2–100 Pa O2 atmosphere. The explosive transformation was caused by UC self-ignition which proceeded as a self-propagating high-temperature synthesis (SHS) reaction through the previously fragmented sample. UC oxidised in air from 873 K to 1173 K showed that better oxide conversion can be achieved at lower temperatures, 873 K, as oxide sintering at higher temperatures, 1173 K, limited further oxidation only on cracked surfaces. Oxide cracking was ascribed to the stresses generated from the volumetric transformation from the carbide to the oxide.