Statistical fracture behavior of doped UO2 using a ball-on-ring equibiaxial flexure test method

Metal oxide dopants, such as titanium and chromium oxides, have garnered considerable attention for their potential to increase grain size (≥ 30 µm) in UO₂ fuel, purportedly enhancing fission gas retention during reactor operation. Fuel performance is significantly impacted by fuel fracture behavior, so it is important to understand the effects of enhanced grain size and dopant content on UO₂ fuel fracture. UO₂ pellets were doped with 0.1 wt% TiO₂ and 0.3 wt% Cr₂O₃ to alter density and grain size. Inductively coupled plasma mass spectroscopy measured dopant levels pre- and post-sintering. X-ray diffraction revealed lattice changes and microstrain via Rietveld refinement. Field emission scanning electron microscopy determined grain sizes of approximately 30 µm for TiO₂ doping and 7 µm for Cr₂O₃ doping. Transverse rupture strength tests were performed on over 30 samples per dataset to obtain characteristic strength and Weibull modulus. Results indicate no statistical difference in fracture strength between 0.1 wt% TiO₂ doped UO₂ and undoped UO₂, while 0.3 wt% Cr₂O₃ doped UO₂ exhibited a 20% decrease in fracture strength. Doped UO₂ samples also showed reduced Weibull modulus compared to undoped UO₂, suggesting increased scatter in fracture strength. This study's findings suggest that titanium and chromium oxide doping in UO₂, regardless of grain size, induce residual stresses, decreasing fracture strength and increasing variability in fracture behavior.