In 2017, nuclear scientists noticed a sudden rise in radioactive ruthenium-106 concentrations over Eurasia. The large plume of material spread quickly. An undeclared nuclear accident had occurred. Nobody was taking the blame for the incident, the biggest since Fukushima in 2011, broadly speaking, the media did not take up the story and the public was not generally made aware of this happening, which may well have had serious implications for the environment and human health. Thankfully, the cloud of material was only slightly radioactive.

Now, a major international research team has analyzed more than 1300 measurements from all over Europe and other parts of the world to determine the source of this ruthenium-106. Their conclusion is that there was no reactor accident, but that the release came from a nuclear reprocessing plant somewhere in the southern Urals. It is presumably no coincidence that the Russian nuclear facility Majak is located in this area. The team also reports that the incident has caused no health risks to the people of Europe.

Georg Steinhauser from the University of Hannover, Germany and Olivier Masson from the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in France, and their colleagues recently reported details in the journal PNAS [Masson, O. et al., Proc. Natl. Acad. Sci. (USA) (2019); DOI: 10.1073/pnas.1907571116].

"We measured radioactive ruthenium-106," explains Steinhauser. "The measurements indicate the largest singular release of radioactivity from a civilian reprocessing plant." In the autumn of 2017, values of up to 176 millibecquerels per cubic meter of air were recorded. These values are almost 100 times higher than the total concentrations measured in Europe after Fukushima. No other radioactive materials were observed, suggesting that the release could only have been a nuclear reprocessing plant.

The informal network of scientists recorded raised ruthenium-106 levels across large parts of Central and Eastern Europe, Asia, the Arabian Peninsula, and even the Caribbean. Data came from 176 measuring stations in 29 countries. Majak was the site of the second-largest nuclear release in history, second to Chernobyl, when in September 1957 a tank containing liquid waste from plutonium production had exploded, causing massive contamination of the area.

Masson and Steinhauser date the 2017 release to some time between 18h00 on 25 September 2017 and 12h00 on 26 September 2017. "It was a pulsed release that was over very quickly," explains Steinhauser; this contrast sharply with the ongoing release over the course of days of radioactive material from Chernobyl and Fukushima.

"We were able to show that the accident occurred in the reprocessing of spent fuel elements, at a very advanced stage, shortly before the end of the process chain," says Georg Steinhauser. "Even though there is currently no official statement, we have a very good idea of what might have happened." It is now known that prior to the release Italian scientists had ordered a source of cerium-144 for neutrino experiments from Majak; this seems too much of a coincidence for the order and the incident not to be connected.