A massive, unexplained cloud of radiation that swept across Europe in 2017 has been traced to one of Russia's largest nuclear facilities, according to NewScientist.

Located between the Volga river and the Ural mountains, the leak coming from the Mayak nuclear development facility released up to 100 times the amount of radiation into the atmosphere as the Fukushima disaster.

Italian scientists were the first to raise the alarm on 2 October, when they noticed a burst of the radioactive ruthenium-106 in the atmosphere. This was quickly corroborated by other monitoring laboratories across Europe. Georg Steinhauser at Leibniz University Hannover in Germany says he was “stunned” when he first noticed the event. Routine surveillance detects several radiation leaks each year, mostly of extremely low levels of radionuclides used in medicine. But this event was different. “The ruthenium-106 was one of a kind. We had never measured anything like this before,” says Steinhauser. -NewScientist

After the radioactivity was detected, the Institute for Radioprotection and Nuclear Security in Paris soon concluded that the most likely source of the leak was the Mayak facility - something Russian officials denied at the time, instead suggesting that the source may have been emissions from a radionuclide satellite battery burning up during re-entry.

The team which tracked down the source of the emission ruled out a satellite because no space organizations reported missing any at the time, and the pattern of radiation in the atmosphere didn't match that of a satellite's reentry.

The report claims that despite being so much higher than the Fukushima release, the radiation level in the Mayak incident wasn't high enough to impact human health.

A nuclear fuel processing facility in Russia looks to have been been the source of the leak

ITAR-TASS News Agency / Alamy Stock Photo

After further investigation of 1300 measurements from hundreds of monitoring stations across Europe, Steinhauser and colleagues found that radiation levels were "between 30 and 100 times higher than those measured after Fukushima."

The leak was unusual because the release was limited to radioactive ruthenium . “If there is a reactor accident, one would expect the release of radioactive isotopes of many different elements,” says Steinhauser. Exactly why such a specific element was released remained a mystery until Steinhauser learned that an Italian nuclear research facility had ordered a consignment of cerium-144 from Mayak before the incident. “There are several indications that the release of ruthenium-106 was linked to this order,” he says. -NewScientist

"This was indeed quite alarming," said Steinhauser.