Identification of a chemical fingerprint linking the undeclared 2017 release of ¹⁰⁶Ru to advanced nuclear fuel...

The undeclared release and subsequent detection of ruthenium-106 (¹⁰⁶Ru) across Europe from late September to early October of 2017 prompted an international effort to ascertain the circumstances of the event. While dispersion modeling, corroborated by ground deposition measurements, has narrowed possible locations of origin, there has been a lack of direct empirical evidence to address the nature of the release. This is due to the absence of radiological and chemical signatures in the sample matrices, considering that such signatures encode the history and circumstances of the radioactive contaminant. In limiting cases such as this, we herein introduce the use of selected chemical transformations to elucidate the chemical nature of a radioactive contaminant as part of a nuclear forensic investigation. Using established ruthenium polypyridyl chemistry, we have shown that a small percentage (1.2 ± 0.4%) of the radioactive ¹⁰⁶Ru contaminant exists in a polychlorinated Ru(III) form, partly or entirely as β-¹⁰⁶RuCl₃, while 20% is both insoluble and chemically inert, consistent with the occurrence of RuO₂, the thermodynamic endpoint of the volatile RuO₄ Together, these findings present a clear signature for nuclear fuel reprocessing activity, specifically the reductive trapping of the volatile and highly reactive RuO₄, as the origin of the release. Considering that the previously established ¹⁰³Ru:¹⁰⁶Ru ratio indicates that the spent fuel was unusually young with respect to typical reprocessing protocol, it is likely that this exothermic trapping process proved to be a tipping point for an already turbulent mixture, leading to an abrupt and uncontrolled release.