U1 Raman band as a signature of uranium interstitials in irradiated UO2

The characterization of irradiated nuclear fuels is challenging because of their high radioactivity. Raman spectroscopy offers a rapid, non-destructive alternative well-suited for studying irradiated nuclear fuels, such as uranium dioxide (UO 2 ). In this study, Au-ion irradiations were performed and the nature of the U1 band was investigated through a combined approach involving Raman spectroscopy, Transmission Electron Microscopy (TEM), and Rate Theory (RT) modeling. Under irradiation, the U1 band area scales with the evolution of interstitial-type dislocation loop density observed by TEM. Under annealing, its decrease matches the reduction in uranium defect concentration predicted by the RT model, with a notable annealing step around 473 K, which we attribute to uranium interstitial-type defect mobility. These observations suggest that the U1 band reflects the presence of 3+ cations in an oxygen cubic cage, that would, in the case of irradiation, correspond to uranium interstitials in a 3+ charged state.