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Transmission electron microscopy characterization of irradiated U-7wt%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-7wt%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-7wt%Mo microstructure starts to undergo grain subdivision and can no longer support the ordered bubble superlattice. The subdivided 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. Introduction: Understanding fuel performance under irradiation is a key aspect to qualifying nuclear fuel. The High Performance Research Reactor Fuel Development program (HPRRFD), formally known as the Reduced Enrichment for Research and Test Reactors (RERTR) program, was tasked to develop a high-density, lowenriched uranium fuel (<20% U 235) to reduce the demand of highly enriched uranium in research reactors for the Global Threat Reduction Initiative (GTRI) [1]. The primary fuel being studied is a U-xMo (x=7-10 wt%) based fuel with the U-7wt%Mo being the primary focus of this paper. The two major fuel forms are U-Mo/Al dispersion fuel and U-Mo monolithic fuel in plate form [2,3]. Dispersion fuels typically contain U-7wt%Mo fuel spheres embedded in an Al-alloy matrix. The Al matrix is often Si enriched to suppress the fuel-matrix interaction (FMI) layer that forms during fuel fabrication and irradiation [2]. Monolithic fuels are primarily U-10wt%Mo foils rolled to a thickness of roughly 250 µm.

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