## 1 Introduction  
Over the last few decades, there has been purposeful effort to phase out the use of high-enriched uranium fuels used in research and test reactors using low-enriched uranium (LEU) fuel [1, 2, 3, 4, 5, 6]. Maintaining reactor performance with the use of LEU dispersion fuels requires an increase in uranium (U) densities in the dispersed fuel particles. Although pure metallic U provides the highest U density, it exhibits anisotropic irradiation growth during irradiation [7], resulting in unacceptable performance relevant to irradiation behavior studies. Alloying U with molybdenum (Mo) results in a metastable body-centered cubic structure that exhibits stable swelling behavior and eliminates anisotropic growth. In particular, U-7 wt.% Mo [8, 9, 10] has been selected as a candidate alloy for dispersion fuel particles. The fuel particles studied here also incorporate a ZrN coating to limit the detrimental effect of the U-Mo interaction with the Al dispersion fuel matrix [11, 12, 13, 14, 15].  
There have been several characterization studies carried out on U-Mo particles irradiated over a range of fission rates, temperatures, and fission densities [2, 16, 17, 18, 19, 20, 21, 22, 14, 23]. As demonstrated in previous work using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), key features observed are the formation of a fission gas bubble superlattice along with decoration of grain boundaries by large fission gas bubbles [12; 19]. The distribution of solid and volatile fission products species has not been well-characterized, however, because of the small scale of precipitates, they are typically assumed to form a dilute solid solutions fission products element with the fuel alloy. Atom probe tomography provides a methodology to characterize spatial localization and elemental composition of materials in three dimensions with near atomic-resolution [24; 25; 26; 27]. This high-resolution information is achieved through ion-by-ion field evaporation from needle-shaped specimens (tip radius typically \(<100\) nm) subjected to high voltage or laser pulses. Moreover, the development of shielded focused ion beam (FIB) instruments presents unique opportunities for site-specific analysis from irradiated nuclear fuels using APT. This method is particularly relevant to understanding the behavior of fission gases, which drive swelling at high burnup, and may compromise fuel integrity [7; 14; 28; 29].  
Our previous APT work in isotopic quantification of uranium showed the value of this technique in analyzing local burnup [30]. In this work, APT is used to characterize the microstructural changes and chemical analysis on fission products formed in fuel during irradiation using a reference fresh fuel specimen and specimens irradiated to intermediate (52%) and high (66%) burnup. Characterization in this work is focused on detailed microstructural studies on fuel before irradiation followed by the distribution of fission gases and solid and volatile fission products through peak identification of the very complex chemistry of the irradiated fuels. APT was also employed to investigate the uniformity of enrichment in as-fabricated or fresh U-Mo samples, which is key for evaluation of the performance of fuel. Since APT has not often been applied to the characterization of irradiated fuel with high burnup, some challenges in this application are also discussed.