Beyond Vegard’s Law: Short-Range Structural Heterogeneity in Gd₂O₃–UO₂ Fuel and Its Impact on Thermophysical Properties

Gadolinium oxide (Gd₂O₃) is widely used as a burnable neutron absorber in uranium dioxide (UO 2 ) fuel, yet the atomic-scale mechanisms governing its impact on thermophysical performance remains elusive. Here, we establish a direct structure-property relationship in Gd-doped UO₂ (mass fractions of 6 % – 15 % Gd), by integrating X-ray diffraction (XRD), scanning electron microscopy (SEM/EDS), synchrotron-based Extended X-ray Absorption Fine Structure (EXAFS), resonant ultrasound spectroscopy (RUS), and thermophysical property measurements. XRD results confirm the formation of a single-phase fluorite solid solution with a linear lattice contraction following Vegard’s law, while EXAFS reveals local distortions, oxygen vacancies, and nanoscale bixbyite-like environments that lack long-range crystallographic order required for detection by XRD. Microstructural analysis shows homogeneous Gd distribution without secondary phases or microcracking with higher dopant levels reducing grain size and increasing porosity. Thermophysical measurements demonstrate that Gd incorporation significantly decreases thermal diffusivity and thermal conductivity, particularly at lower temperatures, due to enhanced phonon–defect scattering. Resonant ultrasound spectroscopy further reveals a progressive reduction in elastic stiffness with increasing Gd content, consistent with defect-induced lattice softening. Thermal resistivity analysis indicates that impurity scattering dominates in Gd-doped UO 2 , while phonon–phonon Umklapp contributions are suppressed with increasing dopant concentration. These results demonstrate the Gd-doped UO 2 is globally single-phase yet locally heterogeneous, and that this short-range structural complexity governs thermal conductivity behavior. The results provide critical experimental insight for atomistically informed fuel performance models and underscore the importance of resolving local atomic environments in the evaluation of doped nuclear fuels.