Si-contamination driven phase evolution in Nd-doped UO₂ porous microspheres

The distribution of neodymium dopant (as surrogate for americium) within the porous network of sintered (U,Nd)O2 microspheres prepared via a sol-gel infiltration technique was comprehensively assessed. Powder X-ray diffraction (XRD) results indicated the formation of a neodymium oxyapatite secondary phase with Nd4.69(SiO4)3O composition, alongside the principal U1-yNdyO2 face-centered cubic phase for y = 20 and 30 mol%. The hexagonal oxyapatite phase was likely formed during sintering under reducing conditions, when volatile SiO from an external contamination reacted with the Nd3+ in the microspheres. Transmission electron microscopy (TEM) analyses on lamellae prepared via focused ion beam (FIB) further confirmed a distinct phase separation between the main (fluorite U1-yNdyO2) phase and the oxyapatite phase, without mutual solubility of Si and U. To gain a deeper insight into the microstructure, energy dispersive X-ray spectroscopy (EDS) and electron probe microanalysis (EPMA) were performed at different locations in cross-sectioned U1-yNdyO2 microspheres. The elemental mappings showed a higher intensity of combined Nd and Si content in the periphery region, where a higher filling of the porosity network is to be expected. XRD lattice parameter analysis was found to be the most reliable method to elucidate the Nd metal fraction y in U1-yNdyO2, as compared to local quantification methods using electron beam induced X-ray fluorescence. The sintered microspheres with a targeted 20 and 30 mol% Nd dopant concentration contain around 3 ± 2 wt% and 5 ± 2 wt% Nd-oxyapatite phase. The formation mechanism of the oxyapatite phase appears to involve capillary infiltration in the liquid state, during high temperature sintering, and it may be of interest for other applications. Despite the phase heterogeneity, the method was successful in infiltrating Nd throughout the porous microspheres at high loading levels, as intended for the fabrication of transmutation targets. The distribution of the Nd-dopant throughout the microsphere was achieved as desired, and this study presents a successful fabrication and detailed characterization of porous uranium oxide microspheres doped with Nd3+, contributing to the development of transmutation target research.