Study on volatilization mechanism of ruthenium tetroxide from nitrosyl ruthenium nitrate by using mass spectrometer

In a cooling malfunction accident of a high-level liquid waste (HLLW) tank, behavior of ruthenium (Ru) attracts much attention, since Ru could be oxidized to a volatile chemical form in the boiling and drying of HLLW, and part of radioactive Ru can potentially be released to the environment. In this study, nitrosyl Ru nitrate (Ru(NO)(NO₃)₃) dissolved in nitric acid (HNO₃), which is commonly contained in a simulated HLLW, was dried and heated up to 723 K, and the evolved gas was introduced into a mass spectrometer. The well-known volatile species, ruthenium tetroxide (RuO₄) was detected in a temperature range between 390 K and 500 K with the peak top around 440 K. Various gases such as HNO₃, nitrogen dioxide (NO₂), nitrogen monoxide (NO) also evolved due to evaporation of the nitric acid and decomposition of the nitrate ions. The ion current of RuO₄ seems to increase with the increasing decomposition of nitrate, while the evaporation of HNO₃ decreases. More volatilization of RuO₄ was observed from the HNO₃ solution containing not only Ru(NO)(NO₃)₃ but also cerium nitrate (Ce(NO₃)₃·6H₂O) which was added for extra supply of nitrate ion, compared with that from the HNO₃ solution containing only Ru(NO)(NO₃)₃. These experimental results suggest that Ru could be oxidized to form RuO₄ by the nitrate ion as well as HNO₃. - Graphical abstract: Ion current intensities of the mass numbers corresponding to O, NO, O₂, NO₂, HNO₃, and RuO₄ obtained in mass spectrometry for dried nitric acid solution containing Ru(NO)(NO₃)₃. Heating rate: 5 K min⁻¹, sample solution weight: 6.61 mg, contained Ru weight: 0.56 mg. The ion current of RuO₄ increases with the increasing decomposition of nitrate, while the evaporation of HNO₃ decreases. - Highlights: • Nitrosyl Ru nitrate (Ru(NO)(NO₃)₃) dissolved in nitric acid (HNO₃) was dried and heated up to 723 K. • Release of ruthenium tetroxide (RuO₄) was detected between 390 K and 500 K by means of the mass spectrometer. • The experimental results suggest that Ru could be oxidized to form RuO₄ by the nitrate ion as well as HNO₃.