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.
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