Microstructure and digestion analysis of UAl and UMn alloys for Medical Isotope Production

Every year, hospitals around the world diagnose and treat 48 million patients with medical isotopes. To this day, the majority of medical isotopes are produced by the fission of high-enriched uranium (235U > 20%) in the forms of UAlx (UAl4, UAl3, and UAl2) targets. This thesis investigates in parallel, how to improve the current UAlx based medical isotope production method and the potential use of the high-density U6Mn (U-density 16.8 g cm-3) alloy to replace the fissile material in next generation low-enriched uranium targets. The oxidation or “digestion” of these materials are studied in basic media to optimize the extraction of isotopes from these fuel types. Part 1 of this thesis studies the digestion of UAlx materials. In UAlx materials, it is found through SEM and energy-dispersive X-ray (EDX) images, that oxidation of triple junctions and intergranular oxidation are key mechanisms in converting UAlx fuel, to UO2 and Na2U2O7 (yellow cake). An aluminum phase present at the grain boundaries is shown to highly enhance oxidation. Electron backscatter diffraction (EBSD) is used to further investigate oxidation at the grain boundaries leading to the discovery of favorable grain boundaries for digestion. Additionally, it is found that smaller grain sizes outperform larger grain sizes in digestion. Part 2 of this thesis studies the digestion of U6Mn. In contrast, U6Mn fuel is resistant to digestion in the same medium and conditions. The use of the oxidant potassium permanganate (KMnO4) is shown to interact with U6Mn particle surfaces to form Na2U2O7. This demonstrates for the first time a successful process that makes this fuel feasible for isotope production. The grain boundary engineering of UAlx fuel can improve currently used isotope extraction processes without adding any additional oxidants, while the use of an oxidant in high-density fuels provides us with a new economically advantageous method for isotope extraction in next generation LEU targets.