  Oxygen defects govern the behavior of a range of materials spanning
catalysis, quantum computing, and nuclear energy. Understanding and controlling
these defects is particularly important for the safe use, storage, and disposal
of actinide oxides in the nuclear fuel cycle, since their oxidation state
influences fuel lifetimes, stability, and the contamination of groundwater.
However, poorly understood nanoscale fluctuations in these systems can lead to
significant deviations from bulk oxidation behavior. Here we describe the first
use of aberration-corrected scanning transmission electron microscopy and
electron energy loss spectroscopy to resolve changes in the local oxygen defect
environment in UO$_2$ surfaces. We observe large image contrast and spectral
changes that reflect the presence of sizable gradients in interstitial oxygen
content at the nanoscale, which we quantify through first principles
calculations and image simulations. These findings reveal an unprecedented
level of excess oxygen incorporated in a complex near-surface spatial
distribution, offering new insight into defect formation pathways and kinetics
during UO$_2$ oxidation.
