The IRSN ruled out an incident in a nuclear reactor, saying it was likely to be in a nuclear fuel treatment site or centre for radioactive medicine. There has been no impact on human health or the environment in Europe, the IRSN said.
IRSN, the technical arm of French nuclear regulator ASN, 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, an IRSN official said.
"Russian authorities have said they are not aware of an accident on their territory," IRSN director Jean-Marc Peres told Reuters. He added that the institute had not yet been in contact with Kazakh authorities.
A spokeswoman for the Russian Emergencies Ministry said she could not immediately comment. It was not immediately possible to reach authorities in Kazakhstan or the Kazakh embassy in Moscow.
Ruthenium 106
Mr Peres said that in recent weeks IRSN and several other nuclear safety institutes in Europe had measured high levels of levels of ruthenium 106, a radioactive nuclide that is the product of splitting atoms in a nuclear reactor and which does not occur naturally.
IRSN estimates that the quantity of ruthenium 106 released was major, between 100 and 300 teraBecquerels, and that if an incident 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.