Transmission electron microscopy characterization of the fission gas bubble superlattice in irradiated U–7 wt%Mo dispersion fuels

Abstract Transmission electron microscopy characterization of irradiated U–7 wt%Mo dispersion fuel were performed on various U–Mo fuel samples to understand the effect of irradiation parameters (fission density, fission rate, and temperature) on the self-organized fission-gas-bubble superlattice that forms in the irradiated U–Mo fuel. The bubble superlattice was seen to form a face centered cubic structure coherent with the host U–7 wt%Mo body-centered cubic structure. At a fission density between 3.0 and 4.5 × 10 21 fiss/cm 3 , the superlattice bubbles appear to have reached a saturation size with additional fission gas associated with increasing burnup predominately accumulating along grain boundaries. At a fission density of ∼4.5 × 10 21 fiss/cm 3 , the U–7 wt%Mo microstructure starts to undergo grain subdivision and can no longer support the ordered bubble superlattice. The sub-divided fuel grains are less than 500 nm in diameter with what appears to be micron-size fission-gas bubbles present on the grain boundaries. Solid fission products typically decorate the inside surface of the micron-sized fission-gas bubbles. Residual superlattice bubbles are seen in areas where fuel grains remain micron sized. Potential mechanisms of the formation and collapse of the bubble superlattice are discussed.