The IRSN - the technical arm of French nuclear regulator ASN – said on Thursday that in recent weeks it and several other nuclear safety institutes in Europe had measured high levels of ruthenium 106, a radioactive nuclide that is the product of splitting atoms in a nuclear reactor and does not occur naturally.

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The IRSN estimated that the quantity of ruthenium 106 released was major, between 100 and 300 teraBecquerels.

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"Measurement results from European stations communicated to the Institute since October 3 have confirmed the presence of ruthenium-106 in the atmosphere of the majority of European countries," IRSN said.

If an accident of this size had happened in France it would have required the evacuation in a radius of several kilometers around the accident site.

The IRSN ruled out an accident at a nuclear reactor, saying it was likely to have come from a nuclear fuel treatment site or radioactive medicine center.

IRSN said there has been no impact on human health or the environment in Europe.

The agency also said in a statement it could not determine exactly where the release of radioactive material took place, but based on weather patterns the most likely zone lay south of the Ural mountains, thus either in Russia or Kazakhstan, an IRSN official said.

"Russian authorities have said they are not aware of an accident on their territory,” IRSN director Jean-Marc Peres told the news agency Reuters.

A spokeswoman for the Russian emergencies ministry said she could not immediately comment, The Guardian newspaper reported.