Mechanistic insights into volatile ruthenium tetroxide formation during evaporation-to-dryness accidents of high-level liquid waste.

Volatile ruthenium tetroxide (RuO4) formed during evaporation-to-dryness accidents (EDA) of high-level liquid waste (HLLW) combines high chemical toxicity with radiotoxicity from isotopes such as 103Ru and 106Ru, making it a primary hazard in nuclear-fuel reprocessing. This study experimentally elucidates the mechanisms governing RuO4 formation while testing the validity of the pragmatic nitrosyl-ruthenium ([RuNO]) surrogate model, an experimental model that assumes Ru in HLLW exists as [RuNO], and evaluates gaseous Ru release using nitric acid solutions of [RuNO], commonly used in RuO4 release assessments by comparing it with the behavior of simulated HLLW (s-HLLW). We systematically investigated potential pathways, including oxidation by liquid-phase nitric acid (HNO3(l)), gas-phase nitric acid (HNO3(g)), and oxygen (O2); thermal decomposition of [RuNO]; and matrix effects of coexisting nitrates. The results identify oxidation by activated HNO3(l), oxidation by HNO3(g) and thermal decomposition of [RuNO] as the principal routes to RuO4, with HNO3(g) playing a far more significant role than previously recognized. Crucially, the RuO₄ release profile from s-HLLW differed markedly from that predicted by the [RuNO] surrogate model, demonstrating that this model fails to capture the complex matrix effects and time-dependent chemical changes of Ru species inherent to real HLLW. These findings have significant implications for improving the accuracy of hazard assessments related to RuO4 release during HLLW EDAs, particularly by highlighting the limitations of commonly used [RuNO] surrogate models.