Additive manufacturing of ultrasonically atomized metallic Zr as a nuclear fuel surrogate

Sammanfattning

Generation IV nuclear systems require advanced fuels like uranium mononitride (UN), but their deployment is constrained by the complex, impurity-sensitive nature of conventional fabrication. To bypass these limitations, this thesis investigates an integrated metallurgical approach to synthesize Zr-based nuclear fuel surrogates. As a preliminary validation of the underlying plasma nitriding mechanism, experiments in a commercial arc melter achieved 59.7% nitridation for Zr sponge, with X-Ray Diffraction (XRD) indicating up to 83% ZrN. Building on this, two distinct manufacturing workflows are explored. The first pathway investigates direct nitride powder synthesis via reactive Ultrasonic Plasma Atomization (UPA) of molten Zr in nitrogen (N2). Scanning Electron Microscopy (SEM) of the resulting powder revealed a mixed morphology of faceted and cracked spheres, dominated by irregular shards. For this atomized powder, IGF and XRD analysis established a nitridation boundary between 45% and 94% ZrN, respectively. The second pathway leverages inert UPA to produce perfectly spherical metallic Zr feedstock for in-situ reactive Laser Powder Bed Fusion (LPBF) in an N2 atmosphere. While SEM of the consolidated LPBF specimens revealed a porous base and a distinct ceramic-colored surface, XRD analysis of the bulk indicated a majority metal composition with no definitive evidence of nitridation. Nonetheless, this study opens pathways for future optimization of the LPBF process to realize unconventional fuel geometries, while concurrently suggesting broader applications of Zr-metal additive manufacturing for advanced nuclear cladding.