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

• Published in: Journal of nuclear materials 2016-10, Vol.479, p.123-129
• Main Authors: Kato, Tetsuya, Usami, Tsuyoshi, Tsukada, Takeshi, Shibata, Yuki, Kodama, Takashi
• Format: Article
• Language: English
• Subjects: High-level liquid waste; Mass spectrometry; Reprocessing; Ruthenium tetroxide; Volatilization
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2016.06.052

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. 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. [Display omitted] •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.