A cloud of radioactive pollution spread over Europe after a possible "accident" at a nuclear facility in Russia or Kazakhstan, French nuclear safety officials confirmed on Friday.
France's nuclear safety institute, IRSN, picked up faint traces of ruthenium 106, a radioactive nuclide that is produced when atoms are split in a nuclear reactor and which does not occur naturally, in three of its 40 monitoring stations late September.
Faint traces were also detected in Germany, Austria, Italy and Switzerland.
There has been no impact on human health or the environment in Europe, a French official stressed, but he added that detection of such a cloud was "absolutely not normal".
At a glance | Nuclear incidents and accidents
IRSN, the technical arm of the French nuclear regulator, said in a statement it could not pinpoint the location of the release of radioactive material but that based on weather patterns, the most plausible zone lay south of the Ural mountains, between the Urals and the Volga river.
This could indicate Russia or possibly Kazakhstan, it said. At the source of the leak, the quantity of ruthenium 106 released was "major", between 100 and 300 teraBecquerels, it said, adding that if an accident of this magnitude had happened in France it would have required the evacuation or sheltering of people in a radius of "a few kilometres around the accident site".
But it said that the probability of importation into France of foodstuffs, notably mushrooms, contaminated by ruthenium 106 near the site of the accident was extremely low.
Measurement from European stations showed high levels of ruthenium 106 in the atmosphere of the majority of European countries, at the beginning of October, with a steady decrease from Oct. 6 onwards.
France picked up traces of the pollution in monitoring stations of Seyne-sur-Mer, Nice and Ajaccio from September 27 to October 13, and has not detected anything since.
Jean-Christophe Gariel, director for health at the IRSN, said that, according to "the data at our disposal, no ruthenium 106 was detected in the UK".
Duncan Cox, leader of Public Health England's radiation emergency response group, said: "Radiation monitors at our sites in Oxfordshire and Glasgow have been checked since September when this substance was reported by other European radiation monitoring institutes, and we have not detected any unusual sources of radiation."
IRSN ruled out an accident in a nuclear reactor. "We observed only ruthenium, which indicates it couldn't come from a nuclear reactor as we would have seen other fission products, like Caesium," said Mr Gariel.
The ruthenium 106 was probably released in a nuclear fuel treatment site or centre for radioactive medicine, he said. Because of its short half-life of about a year, ruthenium 106 is used in nuclear medicine, for example in cancer therapy for eye tumours.
IRSN had initially also thought the radioactive material might have come from a satellite equipped with a thermo-generator containing ruthenium that disintegrated in the atmosphere. "But we don't think this is the case," said Mr Gariel as an IAEA investigation has concluded that no ruthenium-containing satellite has fallen back on earth during this period.
He had contacted his Russian colleagues, and they insisted nothing was amiss.
"From their point of view, they said they had had no problems at all. Rosatom (the Russian nuclear operator) said it had detected nothing." He added that the institute had not yet been in contact with Kazakh authorities.
A spokeswoman for the Russian Emergencies Ministry told Reuters she could not immediately comment.