Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and
structural components. Under this harsh environment, the main factor limiting the service life of
zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process
has recently been linked to the presence of a sub-oxide phase with well-defined composition but
unknown structure at the metal–oxide interface. In this paper, the combination of first-principles
materials modeling and high-resolution electron microscopy is used to identify the structure of this
sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in
Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys.