Oxygen point defect accumulation in single-phase UO2+x

Abstract

$\mathrm{U}{\mathrm{O}}_{2.07}$ was characterized using neutron total scattering in order to elucidate defect morphology in the low oxygen-to-metal regime $(x<0.125$ for $\mathrm{U}{\mathrm{O}}_{2+x})$. Data were collected at temperatures (600 and 1000 \ifmmode^\circ\else\textdegree\fi{}C) coinciding with the single-phase $\mathrm{U}{\mathrm{O}}_{2+x}$ region of the established phase diagram, and results were compared with data of stoichiometric $\mathrm{U}{\mathrm{O}}_{2}$ collected at near-identical temperatures. Experimental data were modeled and interpreted using a holistic approach employing a combination of analyses that characterized multiple spatial length scales. Preferential modeling of long-range atomic arrangements with Rietveld refinement suggests the existence of primarily monointerstitials in $\mathrm{U}{\mathrm{O}}_{2.07}$, whereas preferential modeling of short-range atomic structures with small-box pair distribution function (PDF) refinement indicates the presence of defect clusters in $\mathrm{U}{\mathrm{O}}_{2.07}$. Simultaneous modeling of multiple length scales using complementary reverse Monte Carlo (RMC) and molecular dynamics (MD) methods confirms that excess oxygen atoms in $\mathrm{U}{\mathrm{O}}_{2.07}$ exist as small defects, such as monointerstitials and di-interstitials. RMC and MD results agree with diffraction analysis but differ significantly from small-box PDF refinements, which may be related to a lack of intermediate- and long-range structural information gained from the small-box PDF refinement procedure. Employing a combination of analysis methods with varying length-scale sensitivities enabled more accurate assessment of the $\mathrm{U}{\mathrm{O}}_{2+x}$ defect structure. Our findings provide experimental support for previously predicted di-interstitial defect morphologies in $\mathrm{U}{\mathrm{O}}_{2+x}$ that highly influence the accurate prediction of bulk physiochemical properties of $\mathrm{U}{\mathrm{O}}_{2+x}$, such as oxygen diffusivity.