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

The coronavirus pandemic has highlighted the global nature of threats to our health and economic well-being, emphasising the need for international cooperation and the importance of timely disclosure. As arguments rage over the ultimate source of the virus and who knew what and when, parallels with past radiological incidents are readily apparent. Although hugely overshadowed by the current crisis, the release and atmospheric dispersion of ruthenium-106 (106Ru) across Europe in September 2017 [1] was nevertheless, a serious event. To date, no one has accepted responsibility for the contamination, leaving the scientific community to explore a range of hypotheses. These include, re-entry and combustion of a military surveillance satellite containing a ruthenium isotope battery [2]; an accident at the Mayak Production Association, Russia, during production of a high activity 144Ce source for the European sterile neutrino project [3]; accidental release from a reactor and incineration of a radiotherapy device [1]. An official statement from Rosatom, the Russian Atomic Energy Corporation, in November 2017 raised the possibility of a source in Romania, noting that 106Ru levels detected in that country (176 ± 18 mBq m−3) were higher than those reported in the Russian Federation [4]. This appears at odds with the results of meteorological modelling, including studies by Russian scientists, which suggest an origin in the southern Urals e.g. [5, 6]. The various hypotheses have been assessed by Masson et al [1] who conclude that the most plausible, by far, is an accident at Mayak during reprocessing of relatively ‘fresh’ (~2 y post-irradiation) spent nuclear fuel to recover PBq quantities of 144Ce, causing a release of ~250 TBq 106Ru. However, the Russian authorities have categorically denied that any incidents occurred at their nuclear power plants or fuel reprocessing sites during the period September–October 2017 [4]. 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 106Ru [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 106Ru together with ultra-trace quantities of 103Ru [8, 9]. Neither of these isotopes occurs naturally and their short half-lives (373.6 and 39.3 d, respectively) have allowed the date of release to be narrowed to a small window on 25–26 September 2017.