A radioactive cloud termed "harmless" by French nuclear authorities that moved across Europe in late September and October likely originated in Russia or Kazakhstan.

The makeup of the cloud suggests that it was not from a nuclear power plant accident.

Independent:

The potential nuclear accident that launched the cloud most likely took place between the Volga river and the Ural Mountains in the last week of September, IRSN said.

Ruthenium-106 particles were detected by several countries in concentrations not harmful to human health, according to the agency, but disappeared from France in mid-October.

Jean-Marc Peres, IRSN's director, told Reuters: "Russian authorities have said they are not aware of an accident on their territory," adding that his team had not yet contacted Kazakh authorities.

Germany's Federal Office for Radiation Protection said an accident at a nuclear power plant could be "ruled out" due to the type of particle detected.

It said: "Ruthenium-106 is used as a radiation source in cancer therapy for the treatment of ocular tumours. Ruthenium can also occur during the reprocessing of nuclear fuel elements."

It would be a serious violation of international agreements if any country refused to report the release of any nuclear material into the atmosphere.  That the nation responsible has remained silent suggests that the accident occurred at a military facility.  It could also be a cover-up by the operators of the facility, who don't want the blame.

Whoever is at fault is acting irresponsibly.  While the radioactive cloud is said to pose no immediate threat to human health, those in close proximity to the accident would have been evacuated:

The release of nuclear material was of a scale that, had it occurred in France, would have required the evacuation of or sheltering of people within "several kilometres" of the origin point, IRSN said.

So it wasn't "harmless" for those in the immediate vicinity of the accident.  It raises the question: If the country at fault isn't telling the international community about the accident, did it inform local authorities and evacuate its own citizens to keep them safe?