Oxidation of uranium mononitride and uranium monocarbide

Abstract

The oxidation of uranium mononitride UN has been investigated and some comparative measurements carried out on uranium monocarbide UC. Powdered UN, of surface area 0.2–0.4 m²/g, oxidizes isothermally at a measurable rate in the temperature range 230–270°C. The reaction kinetics are independent of oxygen pressure (50–760 torr) and the apparent activation energy is 30–32 kcal/mole. Complete oxidation leads to an amorphous product of composition UO₃ . N_0.2–0.4 which releases nitrogen only with difficulty and recrystallizes to αUO₃. The oxidation of UC leads finally to UO₃ containing residual CO₂.

X-ray studies of both powdered and single crystalline UN reveal that U₂N₃ and UO₂ are formed as intermediate oxidation products. These develop as a layered structure, with the U₂N₃ sandwiched between the underlying UN crystal and an outer layer of UO₂. Both U₂N₃ and UO₂ are epitaxially oriented with respect to the UN, the f.c. array of uranium atoms being retained. A novel reaction mechanism is proposed to explain these results. This involves lattice diffusion across the product layers with solid/solid reactions taking place at the UO₂/U₂N₃ and U₂N₃/UN interfaces. Experiments with single crystals of UC showed epitaxial orientation of the UO₂ product, but no evidence for higher carbides.