A cloud of radioactive particles has been detected drifting over Europe in the past few weeks. Although it’s now harmless, it’s not entirely clear where it came from, but the spread of material suggests it originated from Russia or Kazakhstan, somewhere between the Ural Mountains and the Volga River.

According to fresh data from France’s Institute of Radiation Protection and Nuclear Safety (IRNS), the radioactive elements contained within the cloud – which is extremely finely dispersed at this stage – didn’t result from a nuclear reactor. It’s more likely than a nuclear fuel treatment site, or a research facility dealing with radioactive medicine, is responsible.

Russia has said that they aren’t aware of any such incidents, and Kazakh authorities haven’t responded yet.

The key ingredient within this mysterious plume of potential doom is ruthenium-106. It’s normally found as a minor component of platinum-containing ores, and only 20 tonnes (22 tons) of the stuff is intentionally produced every single year. Much of it is used to treat small tumors.

Its appearance in the skies of Europe, therefore, is certainly unexpected.

It’s also known to be produced during certain nuclear fission reactions, which is why IRNS suspect that it’s been ejected into the atmosphere as a result of some sort of accident. Although not dangerous to those in Europe, it was released in such a large quantity that several kilometers around the site of the accident – wherever that might be – would have required a quick evacuation.

With a half-life of just a year, however, most of the radiation would have dissipated by now. IRSN have gone on the record as saying that a steady decrease has been occurring since October 6, and that in plenty of places, including France, the radioactive isotope can no longer be detected.

Incidentally, the radiation spike was detected by several agencies last month. Elevated levels were found in Germany, Italy, Austria, Switzerland, and France at the end of September through to the start of October.

At the time, unsubstantiated rumors that Russia had secretly tested a low-yield nuclear weapon in the Arctic did the rounds online, but it was quickly pointed out that such a detonation would produce a plethora of isotopes, not just one. Now we’ve got a better idea of where it came from, even if a more precise locale still remains elusive.

[H/T: Reuters]