A nuclear accident happened somewhere in Russia or Kazakhstan during the last week of September and was kept very quiet, the French nuclear safety institute IRSN has concluded after analyzing a mysterious radioactive cloud that appeared over Europe last month. IRSN researchers say they detected high levels of ruthenium 106, a type of atom that does not occur in nature and is only found when atoms are split in reactors, Reuters reports. The cloud was at its peak during the first week in October and dissipated until it could not be detected in France by Oct. 13, according to IRSN, which says its analysis of weather patterns points to a source between the Ural mountains and the Volga river.
The institute, which says other European nuclear regulators have made similar findings, has ruled out the crash of a radioactive satellite or an accident at a nuclear reactor as the source of the cloud. It says the most likely source was either a facility dealing with spent nuclear fuel or a facility for radioactive medicine. Russian authorities say they're not aware of any accident in the area, while Kazakh authorities haven't been reached for comment yet, IRSN says. The institute says the cloud itself is harmless and is unlikely to contaminate food, though it says there must have been a very large amount of ruthenium 106 released at the accident site, enough that would have theoretically warranted an evacuation of a radius of several miles. (Read more nuclear facilities stories.)