A radioactive cloud drifting across Europe points to accident at a nuclear facility in Russia or Kazakhstan in the last week of September, said the French nuclear safety institute IRSN.

The body said it was unable to pinpoint the precise location of the of the release, but based on weather patterns, the most plausible area lay south of the Ural mountains, between the Urals and the Volga river.

The institute, which is the technical arm of French nuclear regulator ASN, ruled out an accident at a nuclear reactor, adding the cloud of radioactive pollution was more likely to have come from a nuclear fuel treatment site or a radioactive medical centre.

But the body said in a statement yesterday (9 November) the levels of additional radiation in the air recorded in Europe and France "are of no consequence for human health and for the environment".

IRSN director Jean-Marc Peres told Reuters: "Russian authorities have said they are not aware of an accident on their territory." He added that the body had not yet contacted Kazakh nuclear authorities.

IRSN said it considered the quantity of ruthenium 106 released a major incident, with between 100 and 300 teraBecquerels flooding into the immediate atmosphere. The body said if an accident of this size had happened in France, evacuations would be required in a several kilometre-radius around the accident site.

Last month, nuclear scientists in a number of other Europe nations – including Germany, Italy, Austria, Switzerland – also recorded increased in levels of the isotope Ruthenium-106 detected in the atmosphere.

Ruthenium 106 is used in chemotherapy treatment to combat eye tumours, and is also used to help power satellites.