New international research has suggested that an enormous radioactive cloud that covered part of Europe in 2017 was produced by a nuclear fuel reprocessing accident at Russia’s Mayak Production Association, despite repeated denials from Moscow that the facility bears any blame.

The report, published Monday in the Proceedings of the National Academy of Sciences, said further that while the massive release of radiation was not harmful outside of Russia, there may have been more serious fallout in the direct proximity to the site, located near the city of Chelyabinsk near Russia’s border with Kazakhstan.

Yet while the levels of contamination may have fallen within safe limits by the time they reached Europe, the new research suggests that the Mayak leak surpassed the magnitude of the Three Mile Island accident in 1979 – which was previously considered the world’s third worst nuclear power industry accident after Chernobyl and Fukushima.

Russian authorities have repeatedly denied responsibility for the release. Yet the researchers stipulate that the delay in identifying Mayak as its source has robbed the scientific community of vital evidence needed to help prevent other such leaks in the future.

On Tuesday Rosatom, Russia’s state nuclear corporation, issued fresh denials over Mayak’s apparent responsibility for the leak, insisting again that its own investigations had cleared the facility of any mishaps.

The ruthenium leak first made headlines when scientists in Austria reported abnormally high local radiation levels on October 3, 2017. German and French researchers reported similar findings at around the same time. Each of these nations is thousands of kilometers from Mayak, and investigators soon realized that a large swathe of the continent had been affected.

The ruthenium sat above parts of Europe for several weeks before it cleared and radiation levels returned to normal. Researchers from France and Germany quickly concluded that an unspecified Russian nuclear facility near the Kazakh border was the most likely source of the contamination.

Russia’s federal meteorology service eventually acknowledged heighten radiation levels in the area around Mayak more than a month later. But Rosatom, Russia’s state nuclear corporation, denied Mayak’s involvement in the spiking radiation levels. Other highly placed officials in the Russian government rushed to Mayak’s defense, pushing theories that the radiation Europe was detecting had co

A fence surrounding the closed Mayak Chemical Combine. Credit: Courtesty of Alisa Nikulina/Ecodefense

me from crashing satellites.

The new research published this week contradicts all of that. The investigation – conducted jointly by the Institut de Radioprotection et de Sûreté Nucléaire in France, or IRSN, and Leibniz University Hannover in Germany ­ – tracks the contamination directly to Mayak by analyzing 1,300 atmospheric measurements. The researchers were even able to pinpoint when the release occurred: Sometime between noon on September 26, 2017 and noon the following day.

“We are very certain that the source is in the Eurasian border region, and—to the best of our knowledge—there is only one facility that is capable of handling such amounts of radioactivity in this area, and this is Mayak,” George Steinhauser, a nuclear chemist with Leibniz University and one of the report’s lead authors, told Newsweek magazine.

Ruthenium-106 is a byproduct of nuclear fission, which has a half life of 374 days. When spent nuclear fuel is taken out of reactors and reprocessed, these isotopes are typically separated from uranium and plutonium and kept in long-term storage with other radioactive byproducts.

In order to ready ruthenium for storage, it is routinely baked into glass by ovens – a procedure known as vitrification – and Bellona has long maintained that a filtration error during this process might have caused the leak.

During vitrification, which Mayak performs routinely, ruthenium 106 can become volatile and change form, and these changes determine the kind of filter that’s required. Without the right kind of filter, gaseous ruthenium can end up in the atmosphere. This is precisely where not only European researchers but Russia’s weather services, found the ruthenium in the autumn of 2017.

The radioactive cloud was diluted enough that it did not pose a threat to those living under it, though Steinhauser noted they were unable to fully assess the impact in close vicinity of Mayak itself. “If anybody was exposed to the cloud directly, there is a risk of harmful doses,” he told the magazine.

Olivier Masson, Steinhauser’s co-author from IRSN, added that if a similar release had occurred in France “it would have led to the evacuation of workers to avoid any additional over exposure.”

The new report comes at a time when the hit HBO series Chernobyl has made Russian denials of nuclear accidents ring especially hollow to the popular imagination. In that sense, Rosatom’s reaction to the new report from the Proceedings of the National Academy of Sciences was predictable fare.

“We maintain that there have been no reportable events at any Rosatom-operated plants or facilities,” the corporation said. “Both the national regulator and experts from an independent international inquiry inspected the Mayak facility back in 2017 and found nothing to suggest that the ruthenium-106 isotope originated from this site, nor found any traces of an alleged accident, nor found any evidence of local staff exposure to elevated levels of radioactivity.”

Mayak, which is used for both nuclear and civilian purposes, has a long history of nuclear accidents. In 1957, a storage tank at the facility exploded, contaminating more than 200 towns and villages and exposing 272,000 people –a small portion of which were quietly evacuated over the subsequent two years –to radiation. This accident remained a government secret until 1987.

Russian government data now indicate that as many as 400,000 people continue to struggle with continued contamination from the accident, made worse by a legacy inadequate waste handling practices and ongoing official negligence.

In 2004, it was found that the facility had since the 1950s been dumping nuclear waste into the Techa River. Rosatom says that Mayak has since ended such practices.