Radioactive materials might be released into air as a result of the accidental boiling of highly active liquid waste (HALW) in reprocessing plants. In such accidents, volatile radioactive nuclides, such as ruthenium, are released from the tanks into the atmosphere. In addition to the radioactive materials, nitrogen oxides are also released owing to the thermal decomposition of metal nitrates in HALW. The released nitrogen oxides transport volatile ruthenium and cause redox reactions associated with the composition or decomposition of volatile ruthenium. In this study, data of nitrogen oxide release were obtained by heating simulated HALW to 600℃. Data were also obtained for mixed lanthanide solution, mixed lanthanide solution containing zirconium dioxide, and mixed lanthanide solution containing ruthenium dioxide. As a result, the release of nitrogen oxides from the simulated HALW was observed from 200℃ to 600℃, and the main release of nitrogen oxides was observed at about 340℃. All the lanthanide nitrates were found to decompose in the simulated HALW, and the thermal decomposition temperature of the lanthanide nitrates decreased after the addition of ruthenium dioxide to the mixed lanthanide nitrate solution.