  Highly crystalline UO2 nanoparticles (NPs) with sizes of 2-3 nm were produced
by fast chemical deposition of uranium(IV) under reducing conditions at pH
8-11. The particles were then characterized by microscopic and spectroscopic
techniques including high-resolution transmission electron microscopy (HRTEM),
X-ray diffraction (XRD), high-energy resolution fluorescence detection (HERFD)
X-ray absorption spectroscopy at the U M4 edge and extended X-ray absorption
fine structure (EXAFS) spectroscopy at the U L3 edge. The results of this
investigation show that despite U(IV) being the dominant oxidation state of the
freshly prepared UO2 NPs, they oxidize to U4O9 with time and under the X-ray
beam, indicating the high reactivity of U(IV) under these conditions. Moreover,
it was found that the oxidation process of NPs is accompanied by their growth
in size to 6 nm. We highlight here the major differences and similarities of
the UO2 NPs properties with PuO2, ThO2 and CeO2 NPs.
