Scientists just found the 'chemical fingerprint' of an alleged nuclear explosion that went undeclared in Russia

• A group of scientists known as the "Ring of Five" detected unusual levels of radiation in Europe in 2017.
• A new study offers "irrefutable proof" that the radiation came from nuclear waste reprocessing.
• The study lends further evidence to the claim that Russia failed to disclose an accident at the Mayak nuclear facility in September 2017.

For the past three years, a group of scientists called the "Ring of Five" has been inching toward the conclusion that an undisclosed nuclear accident took place in Russia in 2017.

In July 2019, the group released evidence that an explosion may have occurred at the Mayak nuclear facility — 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 behind Fukushima and Chernobyl.

In late 2019, the scientists suggested that, given the large amount of radiation admitted on the date, the accident took place on September 26, 2017. The radiation seemed to spread from Russia's Southern Urals region (where the Mayak facility is located) toward central Europe, Scandinavia, and Italy.

A third study, released Monday, offers "irrefutable proof" that the explosion was linked to nuclear waste reprocessing — a method that separates plutonium and uranium from spent nuclear fuel. The Mayak facility is the largest nuclear reprocessing facility in the region. That makes it the most likely, if not the only possible, origin site — though Russia has never acknowledged a nuclear accident at the facility in 2017.

"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," Georg Steinhauser, a professor at the University of Hanover in Germany and one of the study's authors, told Business Insider in August. "I presume this would not be much different for other superpower nations."

An 'unexpected' discovery in 2017

The Ring of Five has been monitoring Europe's atmosphere for elevated levels of radiation since the mid-1980s. The group originally hailed from five countries: Sweden, Germany, Finland, Norway, and Denmark. But after the Chernobyl disaster in 1986, the team enlisted the help of other nations to expand their efforts. It now includes researchers from 22 countries.

On October 2, 2017, Italian scientists sent an alert to the Ring of Five about elevated levels of ruthenium-106, a radioactive isotope, in Milan. The discovery marked the first time that ruthenium-106 had been found in the atmosphere since Chernobyl.

"We were stunned," Steinhauser said. "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."

Steinhauser said the explosion was the "single greatest release from nuclear-fuel reprocessing that has ever happened."

But Russia has not responded to any findings from the Ring of Five. In December 2017, Russian officials attributed the radiation to an artificial satellite that burned up in the atmosphere. The scientists' latest study excludes that possibility.

'A tipping point for an already turbulent mixture'

The study is the first direct evidence that the ruthenium-106 came from nuclear waste reprocessing. It identified a unique "chemical fingerprint" among samples of the isotope collected in 2017.

Within those samples, the scientists found signs of two chemicals commonly associated with nuclear waste reprocessing: ruthenium(III) chloride and ruthenium(IV) oxide. This provided "direct evidence that fuel reprocessing was the origin of the 2017 environmental release," the scientists wrote.

Under normal circumstances, they added, nuclear facilities would wait at least three years before reprocessing spent nuclear fuel. But in this case, it appears that reprocessing took place after just two years. That means the reprocessing activity was bound to be exothermic, or release heat, according to the study.

"The spent fuel was unusually young with respect to typical reprocessing protocol," the scientists wrote. "It is likely that this exothermic trapping process proved to be a tipping point for an already turbulent mixture, leading to an abrupt and uncontrolled release."

The radiation might not threaten human health

Scientists don't consider the release of ruthenium-106 to be an immediate threat to people's health, but the long-term consequences are unknown. In 2018, 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 in August. The Chernobyl explosion released about 5.3 million terabecquerels (a measurement of radioactivity) of radioactive material into the atmosphere, a 2013 analysis found. The alleged accident at Mayak facility, by contrast, released an estimated 250 terabecquerels of ruthenium.

But Steinhauser said there could be reason to monitor food safety near the Mayak facility 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."