[en] In boiling and drying accidents involving high-level liquid waste in fuel reprocessing plants, emphasis is placed on the behavior of ruthenium (Ru). Ru would form volatile species, such as ruthenium tetroxide (RuO₄), and could be released to the environment with coexisting gases, including nitric acid, water, or nitrogen oxides. In this study, to contribute toward safety evaluations of these types of accidents, the migration behavior of gaseous Ru into the liquid phase has been experimentally measured by simulating the condensate during an accident. The gas absorption of RuO₄ was enhanced by increasing the nitrous acid (HNO₂) concentration in the liquid phase, indicating the occurrence of chemical absorption. In control experiments without HNO₂, the lower the temperature, the greater was the Ru recovery ratio in the liquid phase. Conversely, in experiments with HNO₂, the higher the temperature, the higher the recovery ratio, suggesting that the reaction involved in chemical absorption was activated at higher temperatures. (author)