U3Si2 and UO2 composites densified by spark plasma sintering for accident-tolerant fuels

Abstract This work reports the synthesis and characterization of the U3Si2 and UO2 composites sintered by spark plasma sintering (SPS) with controlled microstructure for accident-tolerant fuel application. The U3Si2 and UO2 composites with different silicide and oxide ratios were synthesized by SPS at temperatures from 1,000 to 1,300 °C for 5 min. The microstructure and phase composition of the SPS densified composite fuels were characterized with scanning electron microscopy, X-ray diffraction (XRD), and energy dispersed spectroscopy (EDS). A systematic study of the thermal and mechanical properties was conducted using microhardness testing and laser flash apparatus, along with oxidation resistance measurements using thermogravimetric analysis (TGA). The results show that the synthesis of composite fuels can be achieved with a 90% theoretical density (TD) at 1,000 °C and over 95% TD when sintered at 1,300 °C. XRD and EDS results confirmed that the dominant phases in the composites are U3Si2 and UO2. Improved physical density generally leads to improved hardness, fracture toughness, thermal diffusivity, and onset temperature during the oxidation process. U3Si2 was found to play a dominant role in determining the mechanical and oxidation properties of the composite fuels, whereas UO2 had a more important impact in controlling the thermal diffusivity of the composites. The composite with 50 wt% UO2 sintered at 1300 °C displayed the onset oxidation temperature of 500 °C by dynamic oxidation testing using TGA at a ramp degree of 10 °C/min. The composite also achieved a high fracture toughness of ∼3.5 MPa m½. These results highlight the potential of composite fuel forms densified by SPS with simultaneously enhanced fissile element density, fracture toughness, thermal transport properties, and oxidation resistance.