An accident of evaporation to dryness by boiling of high-level liquid waste (HLLW) is postulated as one of the severe accidents to occur as a result of the loss of cooling function at a fuel reprocessing plant. In this case, a large amount of ruthenium (Ru) is volatilized and transferred to the vapor phase in the tank, and could be released to the environment. Therefore, the quantitative estimation of the amount of released Ru is one of the key issues in the assessment of the accident consequence. To resolve this issue, an empirical correlation for the rate of Ru transfer to the vapor phase with the temperature, nitric acid mole fraction and activity of HLLW has been developed using the data obtained from the accelerated experiments using simulated HLLW. A simulation study with the developed correlation demonstrated that the estimated amount of Ru transferred to the vapor phase was in good agreement with that obtained from the long-term experiment using actual HLLW.