The oxidation of UC powder was studied in 0.5-6.0 M HNO 3 and HClO 4 , containing 0.01-0.1 M NaNO 2 , 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 HNO 3 was used that contained more than 0.005 M HNO 2 .The dissolution rate increased with increasing HNO 3 concentration and initial solid-to-liquid ratio. The effective dissolution rate constants and reaction orders were estimated for the UC oxidation with H + , NO 3 − and HNO 2 in 0.5-6.0 M HClO 4 . The rate of UC oxidation with HNO 2 exceeded 50-80 times its oxidation rates with NO 3 − ions. During UC oxidation with HNO 2 in 0.5-6.0 M HClO 4 , 60-85% carbon was oxidized to CO 2 . The C/U(VI) ratio in the dissolver solution decreased from 0.5 to 0.1 mg eq/mol with increasing of HClO 4 and HNO 2 solution concentration. Two to five percent of the carbon remained in the insoluble residue of the HNO 3 dissolver 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.