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

Abstract

<jats:p>The undeclared release and subsequent detection of ruthenium-106 (<jats:sup>106</jats:sup>Ru) 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<jats:sup>106</jats:sup>Ru contaminant exists in a polychlorinated Ru(III) form, partly or entirely as β-<jats:sup>106</jats:sup>RuCl<jats:sub>3</jats:sub>, while 20% is both insoluble and chemically inert, consistent with the occurrence of RuO<jats:sub>2</jats:sub>, the thermodynamic endpoint of the volatile RuO<jats:sub>4</jats:sub>. Together, these findings present a clear signature for nuclear fuel reprocessing activity, specifically the reductive trapping of the volatile and highly reactive RuO<jats:sub>4</jats:sub>, as the origin of the release. Considering that the previously established<jats:sup>103</jats:sup>Ru:<jats:sup>106</jats:sup>Ru 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.</jats:p>

Interrogating a Mixed Actinide Basket Using High-Resolution γ-Ray Spectrometry: A Nuclear Forensic Perspective on Possible Smuggling Scenarios

“…For example, in the IAEA database, the majority of the reported incidences involve trafficking of a small quantity of fissile or other radioactive materials, raising concerns about the spreading of radioactivity in the environment. Nuclear forensics, a multidisciplinary branch of analytical science, can play a key role in strengthening safeguard capabilities and ensuring global nuclear security by deterring diversion incidences, even in very small quantities, and providing sufficient evidence on the origin and intended purposes in case of a seizure, that may be useful for legal proceedings. − With the advent of advanced analytical methodologies and improved computational capabilities, the branch of nuclear forensics has witnessed rapid development in the recent past and has been able to solve challenging forensic problems using experiments and artificial intelligence/machine learning techniques. − …”

Capture and immobilization of gaseous ruthenium tetroxide RuO4 in the UiO-66-NH2 metal–organic framework

“…Recent studies have confirmed that the volatile and highly reactive RuO 4 was the origin of the release of the radioactive pollution. [2][3][4] While specific porous adsorbent media, such as activated charcoals or silver exchanged zeolites, have been used for decades for the capture of radiotoxic gaseous iodine ( 129/131 I), 5 the development of a specific process remains much less explored for the immobilization or the separation of gaseous RuO 4 . Typically, in spent fuel reprocessing, gaseous RuO 4 precipitates on cold surfaces as a black nonvolatile RuO 2 deposit, which makes its separation from other radioactive wastes difficult.…”

“…Recent studies have confirmed that the volatile and highly reactive RuO 4 was the origin of the release of the radioactive pollution. 2–4…”

The 106Ru incident of 2017: forensic analysis offers new insights

“…Consequently, and despite strong circumstantial evidence, the case was considered unproven when Richard Wakeford provided an update in the March 2020 issue of this journal [7]. The situation may be about to change following the publication of two recent papers dealing with, respectively, the chemical speciation of deposited 106 Ru [8] and isotopic fingerprinting involving stable ruthenium isotopes [9]. These high sensitivity analyses of filter papers from air sampling stations in Austria, Germany and Sweden confirm the unusual radioactive inventory, which contains only the beta-emitting fission product 106 Ru together with ultra-trace quantities of 103 Ru [8,9].…”