AbstractThe oxidation of UC powder was studied in 0.5–6.0 M HNO3and HClO4, containing 0.01–0.1 M NaNO2, at ambient temperature. The addition of nitrous acid caused the oxidation potential to exceed 400 mV/SCE and thus accelerated the dissolution and increased the dissolution yield to exceed 95%. One hundred percent dissolution at room temperature was achieved within 30 to 40 min when 3.0–6.0 M HNO3was used that contained more than 0.005 M HNO2.The dissolution rate increased with increasing HNO3concentration and initial solid-to-liquid ratio. The effective dissolution rate constants and reaction orders were estimated for the UC oxidation with H+, NO3-and HNO2in 0.5–6.0 M HClO4. The rate of UC oxidation with HNO2exceeded 50–80 times its oxidation rates with NO3-ions. During UC oxidation with HNO2in 0.5–6.0 M HClO4, 60–85% carbon was oxidized to CO2. The C/U(VI) ratio in the dissolver solution decreased from 0.5 to 0.1 mg eq/mol with increasing of HClO4and HNO2solution concentration. Two to five percent of the carbon remained in the insoluble residue of the HNO3dissolver solutions, even when a 100% U(VI) yield was observed. Scanning electron microscopy of the insoluble residue showed the carbon particle size between 1.0 and 5.0 μm.