Phase and defect evolution in uranium-nitrogen-oxygen system under irradiation

Uranium mononitride (UN) with 5 wt.% uranium dioxide (UO₂) is used as a model system to study the phase and defect evolution under proton irradiation in nitride-oxide composite. Phase composition, crystallographic orientation relationships (ORs) and dislocation loops were characterized using X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy techniques. Proton-irradiation at elevated temperatures promoted the transformation of UN into uranium sesquinitride (U₂N₃) and UO₂ phases. U₂N₃ and UO₂ formed a fully coherent structure with two ORs: {002}U₂N₃‖{002}UO₂ and [001]U₂N₃‖[001]UO₂; U₂N₃{101}‖UO₂{101} and U₂N₃[101]‖UO₂[101] due to low lattice misfit (2.3%) and low interfacial energy (127 mJ/m²). Observed oxidation of UN and coherent interface are consistent with density-functional theory calculations which suggest lower energy for oxidized configuration and low energy of the interface. The dislocation loops grew while their number density decreased with the temperature and dose. The loop size was over three times larger in two nitride phases than that in UO₂, while the number density was one order of magnitude higher in UO₂ than in nitride phases. Loop density and diameter were analyzed using a kinetic rate theory that considers stoichiometric loop evolution. This analysis led to the conclusion in all compounds loop growth is governed by mobility of uranium interstitials, and enabled measurement of diffusion coefficients of uranium interstitials and non-metal interstitials and vacancies. This analysis provided a comparative study of early stage of microstructure evolution under irradiation which has implications for use of this mixture as advanced fuel in nuclear energy systems.