Performance and properties evolution of near-term accident tolerant fuel: Cr-doped UO2

Chromium-doped UO ₂ fuel has received significant interest due to the ability for chromium to produce pellets with large average grain size (>30 μm), which has shown to increase fission gas retention during operation. Sintering of chromium-doped UO ₂ pellets was pursued with oxygen potential and sintering atmosphere controlled to tailor the final microstructure of the material. Chromium additions in this study ranged from 750 to 7800 ppm. Cr concentrations were studied pre and post sintering using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Effects of chromium content on lattice parameter and microstructure were examined with X-ray diffraction (XRD) and scanning electron microscopy (SEM). Contraction of the UO ₂ lattice parameter was observed, as well as enlargement of grain size with increasing chromium content up to 4900 ppm Cr ₂O ₃. SEM indicated Cr incorporation within the matrix and the formation of chromium oxide precipitates throughout the microstructure at high Cr concentrations. Evaluation of thermophysical properties of Cr-doped UO ₂ pellets were conducted up to 1200 °C to illustrate their evolution with increased dopant concentration and microstructural changes. The results show that grain size is maximized at 52 μm with Cr ₂O ₃ concentration equal to 4900 ppm; however, grain size decreases at higher Cr ₂O ₃ concentrations. No significant changes were observed in specific heat capacity, linear thermal expansion, and coefficient of thermal expansion compared to undoped UO ₂. The thermal conductivity also decreased through the incorporation of Cr ₂O ₃ dopants above 750 ppm and is shown to be ∼15 % lower than reported UO ₂ values.