In early October 2017 a number of European radiation monitoring bodies began to report spikes in atmospheric radioactivity. The information quickly spread across an informal network of monitoring stations known as the Ring of Five (Ro5). Georg Steinhauser, from the University of Hanover, explains the anomalous nature of this radioactive release was immediately apparent. And, within days, a number of reports from detection laboratories across Europe had verified this considerable, and unusual, event.

"The Ro5 members operate monitoring stations on a routine basis, as the surveillance measurements of the national radiation protection authorities," Steinhauser tells New Atlas. "We did not anticipate any release, but we observed it immediately, when it happened."

The radioactive isotope being detected was ruthenium-106 (106Ru). Not only was the atmospheric presence of this rare isotope highly unusual, but the fact it was measured in isolation was particularly strange. This suggested the leak came from a very specific source, most likely a nuclear reprocessing plant.

"106Ru is a typical signature of nuclear fission waste material (i.e. it is created following the fission of either uranium or plutonium) and is present in spent fuel rods etc," explained Paddy Regan, a nuclear physicist from the University of Surrey, back in late 2017. "If it was a reactor leak or nuclear explosion other radioisotopes would also be present in the 'plume' and from the reports, they are not."

At the time, all eyes quickly focused on the Mayak nuclear processing facility in the southern Ural mountains of Russia. Being one of the few nuclear facilities in the world with the capacity to release such a rare radioisotope many scientists were convinced it must be the source. However, Russian authorities stridently denied any leak could have occurred.

By December 2017 Russian officials had finally admitted detection of ruthenium-106 in the atmosphere surrounding the Ural mountains, but still adamantly claimed it could not have originated from the Mayak facility. Rosatom, Russia's state-run nuclear energy organization, pointed the finger of blame at the possibility of a falling satellite. It was suggested the spike in atmospheric ruthenium-106 must have come from a satellite containing the rare element burning up upon re-entry.

That hypothetical scenario was pretty quickly debunked following an International Atomic Energy Agency investigation concluding no ruthenium-powered satellite had re-entered Earth's atmosphere during the time of the unexplained spike.

A newly published study offers the most convincing evidence to date that the 2017 radioactive spike did indeed originate from the Mayak processing facility in Russia. The study compiled 1,300 measurements from 176 locations across 29 countries. Meteorological modeling allowed the atmospheric contamination levels to be tracked back, revealing the southern Ural mountains to be the most likely point of origin.

Unlike other significant nuclear events, such as Chernobyl or Fukushima, resulting in radiation releases into the atmosphere lasting for days, this particular leak was short and over reasonably quickly. Steinhauser and colleagues can pin the time of the alleged accident to a highly specific window in time: between 6 pm on the 25th of September, 2017 and noon the following day.

So, it is reasonably clear that something happened at the Mayak nuclear facility in that 18-hour window back in late September 2017. An official statement from the nuclear facility in December 2017 insisted nothing unusual occurred to indicate it was responsible for the radioactive leak. Is this a lie, or could something have accidentally occurred at the plant to cause the leak, without anyone noticing it?

"Difficult question," Steinhauser says when asked if the release could have occurred without anyone at the Mayak facility realizing it. "I'd say a release of this magnitude was unlikely to occur unnoticed. It depends on the accident circumstances, but a nuclear facility is usually heavily monitored."

While of major significance, the radioactive cloud that spread over Europe in late 2017 posed no health risk. Levels of ruthenium-106 averaged no higher that 100 to 300 terabecquerels, but a report from IRSN, a French nuclear safety organization, did suggest that a release of this scale would be dangerous for individuals in very close proximity to the leak. It noted that if this occurred in France it would require the immediate evacuation of people, "on a radius of the order of a few kilometers around the location of the release."

So while this radioactive leak was certainly not a major danger to the majority of the population in Europe, it certainly was of international significance. This new study affirms what was previously suspected, that Russia's Mayak facility was the source of the leak, despite the country's continual denials that any nuclear accident occurred.

The new study was published in the journal PNAS.

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