Release Behavior of Gaseous Ruthenium Tetroxide During Heating of High-Level Liquid Waste Simulant During Simulated Accident Conditions

A malfunction of the cooling system of high-level liquid waste (HLLW) and the failure of countermeasures may lead to the evaporation to dryness due to the loss of cooling functions (EDLCF) of the HLLW. In the EDLCF, ruthenium (Ru) can be released at a greater fraction to the initial amount than the other elements in HLLW by forming gaseous Ru. It is important to identify the chemical form of the released gaseous Ru to achieve a comprehensive understanding of the events impacting the source term assessment of Ru in this accident, such as particle formation, gas absorption, and deposition on migration pathways.

In this study, we observed the ultraviolet/visible spectroscopy of the off-gas generated during the heating of a HLLW simulant. Employing a program that allows for the separation and quantification of known components within the spectrum [ruthenium tetroxide (RuO₄), nitrogen dioxide, and nitric acid], we attempted to analyze the composition of gaseous Ru within the generated off-gas. Our findings revealed RuO₄ as the main component of the gaseous Ru in the off-gas after comparing the total amount of released Ru and the RuO₄ released amount obtained via spectroscopic analysis.