[en] Highlights: • UO₂ nanospheres and nanocrystals with diameters near real HBS were obtained via hydrothermal method. • Nanocrystals show a good oxidation resistance in water at room temperature. • A mechanism for synthesizing stable UO₂ nanocrystals has been discussed. - Abstract: The uranium dioxide (UO₂) nanocrystals (NCs) and nanospheres (NSs) with diameters similar to those of the real high burn-up structures (HBS) are successfully obtained by using a hydrothermal method. The nanocrystals showed good oxidation resistance in water at room temperature for more than 50 days, this period is much longer than nanospheres and the previous report (approximately 2 days). The mechanism for the production of stable nanocrystals has also been discussed in detail. Hence, they could be used to develop advanced nuclear fuels.