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.
Peres said that in recent weeks IRSN 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.
IRSN estimates that the quantity of ruthenium 106 released was major, between 100 and 300 teraBecquerels, and 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 several kilometres around the accident site.
The ruthenium 106 was probably released in a nuclear fuel treatment site or centre for radioactive medicine, Peres 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 – but can also be released when nuclear fuel is reprocessed.
The IRSN ruled out an accident in a nuclear reactor, as that would have led to contamination with other substances. It also ruled out the crash of a ruthenium-powered satellite as an IAEA investigation has concluded that no ruthenium-containing satellite has fallen back on Earth during this period.
Measurement from European stations showed relatively high levels of ruthenium 106 in the atmosphere of the majority of European countries at the beginning of October, with a steady decrease from 6 October onwards. The radioactive element has not been detected in France since 13 October.
The IRSN said that the concentrations of ruthenium 106 in the air that have been recorded in Europe were of no consequence for human health and the environment. Monitoring stations in Germany, Austria, Italy and Switzerland all detected very low levels of ruthenium 106 from late September. Seven German stations recorded levels from a few microBecquerels to five milliBecquerels per cubic metre of air, posing no hazard to health.
The institute also said that the probability of importation into France of foodstuffs, notably mushrooms, contaminated by ruthenium 106 near the site of the accident is extremely low.
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