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Radiation detectors across Europe began to light up recently, after detecting a cloud of radioactivity wafting over parts of the continent.

It was caused by an isotope called Ruthenium-106 and researchers were quick to reassure the public that the levels wouldn't cause harm.

But the mystery has continued, with many left wondering where the cloud came from and whether it could be a sign of a larger, unreported nuclear accident.

Some are pointing fingers at Russia. Authorities there have confirmed reports of a spike in radioactivity in the air over the Ural Mountains, but the suspected culprit — a Mayak plant where there have been previous nuclear accidents — has denied it was the source of contamination.

A member of Greenpeace Germany's anti-nuclear program takes a radiation reading on the banks of the Techa river in the village of Muslyumovo on Nov. 17, 2010. (Denis Sinyakov/Reuters)

Sven Poul Nielsen is a researcher at Denmark's Center for Nuclear Technology. He spoke with As It Happens host Carol Off about this recent radioactivity.

This Mayak factory, is [Ruthenium-106] believed to be one of its products?

They claim they stopped producing Ruthenium-106 ... so what can we say. When they say they are not the source, then we don't know.

Can it be naturally occurring?

No. That is completely out of the question.

Ruthenium-106 is a fission product — a one-year radioactive half life. It can only be released alone if chemical separation has been applied to the fission product from the spent nuclear fuel.

Detection of <a href="https://twitter.com/hashtag/Ruthenium?src=hash&ref_src=twsrc%5Etfw">#Ruthenium</a> 106 in France and in Europe in late September: Results of IRSN’s investigations and updated information on the plausibility of the origin of the release <a href="https://t.co/Y8uzAxPnpU">https://t.co/Y8uzAxPnpU</a> <a href="https://t.co/OP14AjwE5L">pic.twitter.com/OP14AjwE5L</a> —@radioprotection

Does it pose a health risk to people in the region?

Well, of course. If the concentrations are high enough, then any radioactive element isotope would be a health risk. At the source location, the concentration, or the amounts of radioactivity, have been sufficiently high to cause a health risk depending on the circumstances.

But in Europe, where the ruthenium was observed at first, the levels were so low so they didn't pose a health risk to anybody.

Do you know if Russian authorities have warned people in the Mayak region or in the Ural Mountain area where this cloud is coming from? Have they been warned of health risks?

No. I haven't seen any reports of that.

What do you make of that?

It's strange. All the observations which have been used by methodologists to run backwards calculations … indicate that the source should be somewhere in the Chelyabinsk region around the Ural mountains. But the Mayak enterprise claim they're not the source so we simply don't know.

Perhaps they retained some of these secret attitudes even today. - Sven Poul Nielsen

But something doesn't add up, does it?

Yes. The most likely explanation is that the Mayak enterprise would be the source of the release. But since they claim they're not the source … we don't know anything more than that. So, that's where we are now.

A man fishes in a lake that connects to the nearby Techa River in April 2016, near the village of Muslyumovo, Chelyabinsk region in Russia, which is polluted with radioactive waste from the Mayak nuclear plant. (The Associated Press)

Has there ever been a problem with the Mayak factory before?

The Mayak factory is the place where the Russians back in time produced the nuclear materials for weapons — for nuclear weapons. It is, I suppose, a very secret area back in time until the Soviet Union broke down. So perhaps they retained some of these secret attitudes even today.

This transcript has been edited for length and clarity. To hear more, listen to the audio above.

With files from the Associated Press.