Fluorination of niobium compounds with fluorine for fluoride volatility method

Niobium is one of the fission products contained in spent nuclear fuel. Since niobium pentafluoride (NbF₅) has high volatility, it is considered that niobium volatilizes with uranium hexafluoride when applying the fluoride volatility method, which is a promising pyro-reprocessing method. In this study, fluorination behavior characteristics of niobium compounds, such as reaction temperature, volatility, and reaction path, were investigated by thermogravimetric and differential thermal analyses and X-ray diffraction analysis. The target compounds were niobium metal, the niobium oxides NbO, Nb₂O₃, NbO₂ and Nb₂O₅, and niobium oxyfluoride (NbO₂F). All the niobium compounds reacted exothermically and were volatilized completely by the reaction with F₂. It was considered that niobium volatilized as NbF₅. The fluorination reactions started respectively at 180, 200, 300, and 300 °C for niobium metal, NbO, NbO₂ and Nb₂O₅. In the fluorination of niobium oxides, the intermediate product NbO₂F was also fluorinated above 300 °C and volatilized completely. Nb₂O₃, which seemed to be a mixture of NbO and NbO₂, reacted with F₂ as described by the summation of the fluorination reactions of NbO and NbO₂. The reaction mechanism for the fluorination of niobium compounds obtained in this study is applicable to evaluation of the niobium transfer phenomena in the reprocessing process of the fluoride volatility method.