Improved doping and densification of uranium oxide microspheres using starch as pore former

A hybrid route combining internal gelation and a single-step infiltration was investigated to prepare U1-yNdyO2-x sintered microspheres as a surrogate for U1-yAmyO2-x transmutation targets. This simplified procedure eliminates the use of successive infiltration and re-calcination steps to reach higher dopant concentrations. The use of starch as a pore-forming agent in the internal gelation process for fabricating porous uranium oxide microspheres has been studied in detail to improve the conditions for an efficient infiltration with dopant solution. While the crystalline structure and composition of dried microspheres was not significantly affected by the process conditions, the onset of the UO3 to U3O8 phase transformation during calcination is shifted from 803 K to 823 K when starch was used. Biphasic mixtures of β-UO3 and α-U3O8 were formed when calcination temperatures between 833 K and 853 K were applied. High accessible porosity levels (26 - 33 %) were measured after calcination, and this resulted in efficient infiltration behavior allowing to reach average dopant levels up to y = 30 mol% after sintering. Microstructural features of the sintered microspheres (grain size, porosity distribution, dopant homogeneity) are discussed.