Feasibility and Process Mechanism of Sodium Reduction of Uranium Hexafluoride

The management and resource utilization of depleted uranium hexafluoride (DUF₆), a massive stockpile generated by the nuclear industry, represent a critical challenge in the nuclear fuel cycle. Conventional reduction strategies typically involve a two-step process: first reducing DUF₆ to solid uranium tetrafluoride (UF₄) using hydrogen (H₂), followed by de-fluorination to metallic uranium using strong reductants such as calcium (Ca) or magnesium (Mg). This conventional route suffers from lengthy multi-stage operations, high energy consumption, and the inevitable introduction of extrinsic impurities during complex material handling, which significantly increases separation costs and limits product purity. To address these bottlenecks, in this study, the sodium thermal reduction method, which has great potential for application, was selected to carry out the reduction research of UF₆. Characterized by its high reactivity, this process promises the efficient conversion of DUF₆ to metallic uranium via a single-step or highly streamlined pathway. A systematic research framework integrating “thermodynamic evaluation-reaction pathway analysis-kinetic optimization” was established to elucidate the fundamental mechanisms of the direct exothermic reaction between uranium hexafluoride (UF₆) and metallic sodium (Na). Specifically, first, the spontaneous tendency and feasibility boundary of the reaction were theoretically defined by calculating key thermodynamic parameters. Subsequently, within the thermodynamically feasible range, multiple potential stepwise reduction pathways were constructed based on first-principle calculations, and the energy barriers along with energy changes for each elementary step were analyzed. Finally, kinetic modeling was performed on the identified rate-determining step (RDS) to quantify the effect of temperature on the reaction rate constant. The results indicate that the sodium thermal reduction of UF₆ is thermodynamically spontaneous below 4 816 K, and the driving force decreases with increasing temperature. Microscopic kinetic analysis further reveals that the reaction proceeds most favorably via the mononuclear cluster intermediate pathway (UF₆→*UF₅→*UF₄→*UF₃→*UF₂→*UF→U), wherein the transformation from *UF₃ to *UF₂ corresponds to the RDS of the overall reaction. Kinetic modeling of this step clarifies the quantitative relationship between temperature and the reaction rate constant, revealing a pattern of “exponential growth in the low-temperature range and a diminishing increase in the high-temperature range”. In practical operations, the selection of reaction temperature must balance energy consumption and reaction rate. In summary, this study reveals the reaction mechanism of ultrafiltration sodium thermal reduction at the atomic scale. These findings can provide theoretical support for the development of new reduction processes with simplified processes, minimal impurity generation, and high economic feasibility, thereby providing sustainable solutions for the stabilization of depleted uranium resources.