Study on the Oxidation Behavior of Uranium Dioxide in Carbonate Molten Salts

The molten salt oxidation (MSO) process has significant potential for spent fuel reprocessing. In this work, we focused on the initial oxidation stage of the MSO process, investigating the effects of temperature, KNO 3, and carbonate molten salt composition on the oxidation behavior of UO 2 in molten salt. The results indicate that increasing the molten salt temperature can significantly enhance the oxidation rate of UO 2; the oxidation rate of UO 2 is fastest in the Li 2 CO 3 –K 2 CO 3 eutectic molten salt for different molten salt compositions; the addition of KNO 3 has a significant impact on the oxidation process of UO 2 ─when the molar ratio of KNO 3 to UO 2 exceeds 0.4, complete oxidation of UO 2 in carbonate molten salt can be achieved within 1 h. In addition, the oxidation product evolution during UO 2 oxidation was investigated. The results show that UO 2 is first oxidized to U 3 O 8 and then is further converted to K 2 U 2 O 7 . At temperatures above 650 °C in the presence of carbonates, K 2 U 2 O 7 is transformed into Li 4 UO 5 . In the end, we found that KNO 3 functions as an oxidant in the UO 2 oxidation process, deduces the UO 2 molten salt oxidation equation, and summarizes the reaction mechanism.