Russia’s weather service has confirmed there were “extremely high” concentrations of radioactivity in several parts of the country in late September.
A French nuclear safety group had previously reported a cloud containing the radioisotope Ruthenium-106 drifting over Europe in October.
European authorities said the cloud had likely originated in the southeastern Urals region of Russia or Kazakhstan.
The Russian meterological service did not point to any specific source of the pollution. It said the highest concentration was registered in Argayash, about 30 kilometres from the Mayak nuclear facility.
In 1957, Mayak was the site of one of the worst nuclear accidents in history. Today, it is a reprocessing site for spent nuclear fuel.
The French report ruled out a nuclear reactor accident as the source of the Ru-106 since other radioactive elements would also have been detected. It said the concentrations measured in Europe were not a danger to public health.
Greenpeace Russia has called for an in-depth inquiry into the incidents at the Mayak facility.