Supercritical hydrothermal synthesis of UO2+x: stoichiometry, crystal shape and size, and homogeneity observed using 23Na-NMR spectroscopy of (U, Na)O2+x

The hydrothermal synthesis of pure uranium dioxide under supercritical water (SCW) conditions was investigated using a starting material composed of a uranyl(VI) nitrate solution at 450 °C. The synthesis was conducted using an autoclave method for 30 min and yielded UO2+x particles with nonstoichiometries, shapes, and sizes depending on additives such as ethanol, ammonium carbonate, guanidine carbonate, and several aldehydes. The nonstoichiometry, i.e., the value of x in UO2+x, varied from 0.03 with ethanol to 0.20 with a combination of ethanol and guanidine carbonate. The crystallite size was the largest (57 nm) with ethanol and methanol, and the smallest (37 nm) with propionaldehyde. Additionally, the morphology of the UO2+x particles was modified from a spherical shape with a diameter of 0.20 μm to a rectangular parallelepiped shape with a size of 2.25 μm × 0.50 μm. The 23Na-NMR spectra of the Na-doped UO2+x samples indicated high homogeneities in the products prepared using SCW hydrothermal synthesis. The SCW hydrothermal synthesis may be a promising method for producing homogeneous UO2+x and its solid solutions with well-defined nonstoichiometries (x), shapes, and sizes.