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, KNO₃, and carbonate molten salt composition on the oxidation behavior of UO₂ in molten salt. The results indicate that increasing the molten salt temperature can significantly enhance the oxidation rate of UO₂; the oxidation rate of UO₂ is fastest in the Li₂CO₃–K₂CO₃ eutectic molten salt for different molten salt compositions; the addition of KNO₃ has a significant impact on the oxidation process of UO₂─when the molar ratio of KNO₃ to UO₂ exceeds 0.4, complete oxidation of UO₂ in carbonate molten salt can be achieved within 1 h. In addition, the oxidation product evolution during UO₂ oxidation was investigated. The results show that UO₂ is first oxidized to U₃O₈ and then is further converted to K₂U₂O₇. At temperatures above 650 °C in the presence of carbonates, K₂U₂O₇ is transformed into Li₄UO₅. In the end, we found that KNO₃ functions as an oxidant in the UO₂ oxidation process, deduces the UO₂ molten salt oxidation equation, and summarizes the reaction mechanism.