A group of scientists called the 'Ring of 5' found evidence of a major nuclear accident that went undeclared in Russia

• In 2017, a group of scientists known as the "Ring of Five" detected "an unprecedented release" of radiation in Europe and Asia.
• At the time, no country claimed responsibility for the release, but a new study from the Ring of Five attributes it to a nuclear accident at Russia's Mayak nuclear facility.
• The facility was previously the site of the 1957 Kyshtym explosion, the world's third-worst nuclear accident, behind Fukushima and Chernobyl.
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A group of scientists called the "Ring of Five" has been scouring Europe's atmosphere for elevated levels of radiation since the mid '80s.

In July, the group released a study detailing evidence of an undisclosed nuclear accident that may have taken place less than two years prior. The likely culprit, the scientists said, was the Mayak nuclear facility in Russia, which was once the center of the Soviet nuclear-weapons program.

At the time of the alleged accident in 2017, Russian officials said the facility wasn't the source of the release, even though the nation showed elevated levels of a radioactive isotope called ruthenium-106. Instead, officials in Russia attributed the radiation to an artificial satellite that burned up in the atmosphere.

But the latest Ring of Five study contradicts that account. Their research traced the source to an area of Russia known as the Southern Urals. The scientists also figured out that the release came from a nuclear reprocessing facility, which separates plutonium and uranium from spent nuclear fuel.

Georg Steinhauser, a professor at the University of Hanover in Germany and one of the study's lead authors, said Mayak is the most likely place of origin because it's the largest nuclear reprocessing facility in the area. The facility was the site of the 1957 Kyshtym explosion, the world's third-worst nuclear accident behind Fukushima and Chernobyl.

Scientists 'were stunned' to find evidence of a nuclear accident in Russia

After the Chernobyl disaster sent plumes of radioactive material spiraling across Europe in 1986 , the scientists in the Ring of Five - who hailed from Sweden, Germany, Finland, Norway, and Denmark - enlisted the help of other nations to expand their efforts. The group now includes researchers from 22 countries.

The team first detected what they called "an unprecedented release" of ruthenium-106 in the atmosphere in Europe and Asia in 2017. The discovery marked the first time that ruthenium-106 had been found in the atmosphere since Chernobyl. Even the 2011 nuclear meltdown at Fukushima didn't release detectable levels of that isotope.

"We were stunned," Steinhauser told Business Insider. "We are measuring the air 24/7, 365 days a year, and suddenly we came up with something unusual and unexpected."

For almost two years, the scientists traced the pathway of the radioactive isotope back to its original source by modeling atmospheric conditions such as altitude, wind direction, and the shape of the plumes.

Ultimately, they determined that all evidence pointed to the Mayak facility. Russia hasn't issued a response to the finding.

The 'single greatest release from nuclear-fuel reprocessing' ever

The scientists don't consider the levels of radiation they detected to be an immediate threat to people's health, but the long-term consequences are unknown. Last year, France's Institute for Radiological Protection and Nuclear Safety determined that the levels of ruthenium-106 in the atmosphere do not pose danger to human health or the environment.

The nuclear release was "nothing compared to Chernobyl," Steinhauser said. But he noted that it was still the "single greatest release from nuclear-fuel reprocessing that has ever happened."

One unanswered question, he said, is whether the population near the Mayak facility ingested any radiation in their lungs. Steinhauser also said there could be reason to monitor food safety if radiation leaked into the soil and water.

"I'm not blaming Russia, because certain types of accidents are difficult to spot," he said. "For me, it is about the lessons to be learned."

After Fukushima, he said, Japanese officials shared information about the accident that helped improve the world's safety regulations for nuclear power. In the wake of that disaster, the European Union began to require "stress tests" to evaluate the stability of nuclear reactors.

Steinhauser said the Ring of Five was "hopeful that Russia would have come forward" in 2017 in the same way Japan did in 2011. By revealing the mistakes that lead to the accident, he said, Russia could help make nuclear power safer than it was before.