An incident in September-October at a cold war nuclear disaster site in Russia is suspected to be the source of extremely high concentrations of radioactive material that spread over Europe, AFP reported on Monday.

Reports of a “radioactive cloud” first surfaced on November 9, when the Institute for Radioprotection and Nuclear Safety in Paris said ruthenium-106 from Russia’s direction was detected in France between September 27 and October 13. Ruthenium-106 is a product of splitting atoms in a reactor. It is used in certain medical treatments and does not occur naturally.

The institute said the source of the pollution was an accident somewhere between the Volga River and the Ural Mountains, but added that in Europe, the concentrations measured were not a danger to public health.

When this report came out, Russia’s nuclear corporation Rosatom said “radiation around all objects of Russian nuclear infrastructure are within the norm and are at the level of background radiation”. In mid-October, it issued a statement saying “in samples tested from September 25 to October 7, including in the southern Urals, no trace of ruthenium-106 was found, except in Saint Petersburg”. On Monday, however, Russia’s meteorological service confirmed “extremely high” concentrations of the radioactive isotope in several parts of the country in late September, AFP reported.

“Probes of radioactive aerosols from monitoring stations at Argayash and Novogorny were found to contain radioisotope Ru-106 between September 25 and October 1,” the Rosgidromet service said.

The French institute said a nuclear reactor could not have been the source of the Ru-106 as other radioactive elements would also have been detected. It suggested instead that a discharge from an installation linked to the nuclear fuel cycle or something that produced radioactive materials was the cause of the radioactive cloud.

Where the isotope was detected

The highest concentration was registered at the station in Argayash, a village in the Chelyabinsk region in the southern Urals. Here, the levels of Ru-106 exceeded the natural background pollution by 986 times, the Russian service said.

The isotope was then detected in Tatarstan, followed by southern Russia, eventually reaching “all European countries starting in Italy and toward the north of Europe” from September 29, Rosgidromet said. The Argayash station is 30 km from the Mayak nuclear facility, which in 1957 was the site of one of the worst nuclear disasters in history.

After Russia’s confirmation, Greenpeace Russia called on Rosatom to open “an in-depth inquiry and publish the results about the incidents at Mayak”.