In reprocessing facilities, a potentially severe accident, known as the “evaporation-to-dryness accident (EDA),” raises substantial concerns regarding the release of radioactive materials from high-level liquid waste (HLLW). In particular, ruthenium (Ru) forms gaseous compounds and is released at a higher ratio than other elements; thus, an accurate understanding of its release behavior and chemical structure during such accidents is critical for facility safety design and impact assessment.
 In this study, we developed a nondestructive method for analyzing the gaseous Ru compounds released from a heated simulant of HLLW (s-HLLW) via ultraviolet-visible spectroscopy to identify the chemical structure of these compounds. Results revealed that approximately 90 % of the released Ru was in the form of ruthenium tetroxide (RuO₄). The findings of this study provide critical insights into the mechanisms of gaseous Ru formation and release during accidents, contributing to the refinement of accident impact assessment models. Moreover, by identifying RuO₄ as the primary species responsible for Ru release, this study enables development of removal and containment technologies with improved effectiveness as accident mitigation measures.
 In addition, the developed nondestructive analytical method can be applied to more detailed evaluations of the Ru compound release behavior and composition during EDA accidents, thereby contributing to improved safety measures at reprocessing facilities.