Russia appears to have kept a major nuclear accident secret. But scientists called the 'Ring of 5' tracked the plume of radiation to its source.

• In 2017, a group of scientists known as the "Ring of Five" detected "an unprecedented release" of radiation in Europe.
• Two years later, they figured out that the release likely came from an undeclared nuclear accident at Russia's Mayak nuclear facility.
• A new study shows that most of that radiation was emitted on September 26, 2017.
• According to the scientists, the plume spread from the Mayak facility toward Central Europe, reaching Italy on October 2, 2017.
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A group of scientists called the "Ring of Five" noticed something unusual in the atmosphere in late 2017: Air across Europe showed "unprecedented" levels of the radioactive isotope ruthenium-106.

The isotope is often made when reprocessing nuclear fuel.

"We were stunned," Georg Steinhauser, a professor at the University of Hanover in Germany who is part of the group, told Business Insider in August. "We did not have any anticipation that there might be some radioactivity in the air. We were just measuring air filters as we do on a weekly basis, 52 times a year, and suddenly there was an unexpected result."

The Ring of Five, which had been monitoring Europe's atmosphere for elevated levels of radiation since the mid '80s, spent the next two years looking for the cause of the spike.

The culprit, according to a study released in July, was an undisclosed nuclear accident at the Mayak nuclear facility in Russia, which was once the center of the Soviet nuclear-weapons program. Mayak was also the site of the 1957 Kyshtym explosion, the world's third-worst nuclear accident. More than 10,000 nearby residents were forced to evacuate at the time.

Russia has never acknowledged that any nuclear accident happened at the Mayak facility in 2017, and has not responded to any findings from the Ring of Five.

But now, the scientists have unraveled the mystery even further. A second study published last month offers even more evidence that an accident occurred at Mayak in 2017. It even pinpoints a timeline: Most of the ruthenium was emitted on September 26, 2017.

Tracing a radioactive plume across Europe

The Ring of Five is so named because the group was originally made up of scientists from five nations - Sweden, Germany, Finland, Norway, and Denmark - but it now includes researchers from 22 countries. Their monitoring work takes takes place 24 hours a day, 365 days a year.

The new study suggests that the Mayak facility likely released 250 terabecquerels (a measurement of radioactivity) of ruthenium into the atmosphere. The Kyshtym explosion, by comparison, released around 2,700 terabecquerels of ruthenium. The world's worst nuclear accident, Chernobyl, released around 5.3 million terabecquerels of radioactive material, according to a 2013 analysis.

To find out where the 2017 radioactive plume came from, scientists traced the path of the wind at the time using more than 1,100 measurements from the fall of that year. That required studying the wind's altitude and direction, as well as weather conditions that may have changed its course.

The scientists determined that the plume started out in the Southern Urals, where the Mayak facility is located, then was driven towards southwest Russia. It arrived in Romania on September 29, then split in two.

The main part of the plume spread toward Central Europe, where it encountered rain in Bulgaria. Plant and soil samples taken in the country showed elevated levels of ruthenium at the time. After that, the plume moved north to Scandinavia before arriving in Italy on October 2, 2017. That day, Italian scientists sent an alert to the Ring of Five about elevated levels of ruthenium in Milan.

Steinhauser called this the "single greatest release from nuclear-fuel reprocessing that has ever happened."

Russia has not responded to the Ring of Five's findings

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

Russia still hasn't issued a response to either of the studies the Ring of Five published this year.

"We should not forget that Mayak is a military facility - and, of course, the Russian Federation is very reluctant when it comes to talking about military facilities," Steinhauser said. "I presume this would not be much different for other superpower nations."

The scientists don't consider the levels of radiation they detected to be an immediate threat to people's health. 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. But the long-term consequences are unknown.

Another unanswered question, Steinhauser said, is whether the population near the Mayak facility breathed any radiation into their lungs. He added that there could be reason to monitor food safety if radiation leaked into the soil and water.

"We would like to get some more in-depth information on what actually happened," he said. "There's a good chance that we'll catch every single accident - but, in the present case, surprise was on our side."