A 10 MW high-temperature gas-cooled reactor (HTR-10) has been constructed in China. It is important to study reprocessing of spent nuclear fuel from high-temperature gas-cooled reactor (HTGR) for recovery of nuclear resource and reduction of nuclear waste. Ceramic UO2 pellets of 0.5 mm in diameter for HTR-10 fuel elements were converted into nitrate with N2O4. When the molar ratio of water to uranium was 6, the conversion efficiency was above 99% after 180 min at 80 degrees C, and the conversion product was a mixture of UO2(NO3)(2)center dot 3H(2)O and UO2(NO3)(2)center dot 6H(2)O. When ceramic UO2 pellets were transformed to U3O8 powder with a particle size below 100 mu m under an O-2 stream and high temperature, the U3O8 powder was quickly converted into nitrate with N2O4 within 45 min. Water showed a significant effect on the reaction of U3O8 powder with N2O4, whereas other operation conditions had little effect on the reaction.