A station close to the Mayak nuclear facility in the Chelyabinsk region detected "extremely high pollution" of the radioactive isotope Ru-106, Russian meteorologists said yesterday.
But a representative of Rosatom nuclear corporation told AFP "there have been no incidents at nuclear infrastructure facilities in Russia," adding that the concentration detected posed little threat.
The Mayak facility in the southern Urals, which is under Rosatom's umbrella, also said the contamination "has nothing to do with Mayak's activities".
Mayak was the site of one of the worst nuclear disasters in history in 1957.
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In what is known as the Kyshtym disaster, an explosion at Mayak broke a container holding radioactive waste, prompting the evacuation of nearly 13,000 people from the area.
Russia's consumer watchdog Rospotrebnadzor also said the levels registered by the weather service were safe, giving a different figure of "200 times below" a level that would be considered unacceptable.
"An emergency discharge of ruthenium could be connected with the process of nuclear waste vitrification," the Russian arm of the NGO said.
"Another possibility is that materials containing ruthenium-106 were placed in a metal remelting furnace. Both these activities take place in the Rosatom complex at Mayak."
Ruthenium-106, which is produced by splitting atoms in a reactor, is used in certain medical treatments. It does not occur naturally.
National weather service chief Rosgidromet Maksim Yakovenko also said the concentration detected "poses no danger to population's health", adding that it was not the organisation's job to detect the source.
It said that the source of the pollution was probably an accident somewhere between the Volga River and the Ural Mountains, adding that the concentrations measured in Europe were not a danger to public health.
It added that the quantity of Ru-106 at the suspected release site was "between 100 and 300 teraBecquerels" which is "very high" and in France would have led to measures to protect the population in the area.