Mayak.

This is coolbert:

Finally! Culprit has been found! Rosatom! Gigantic!

From the Internet web site New Scientist and the article by Ruby Prosser Scully we have a description of mystery solved!

"Gigantic, mysterious radiation leak traced to facility in Russia"

"A nuclear fuel processing facility in Russia looks to have been been the source of the leak"

"The source of a gigantic, mysterious leak of radioactive material that swept across Europe in 2017 has been traced to a Russian nuclear facility, which appears to have been preparing materials for experiments in Italy."

"The leak released up to 100 times the amount of radiation into the atmosphere that the Fukushima disaster did. Italian scientists were the first to raise the alarm on 2 October, when they noticed a burst of the radioactive ruthenium-106 in the atmosphere. This was quickly corroborated by other monitoring laboratories across Europe."

"Mysterious" leaks of radioactive material into the atmosphere hardly a novel occurrence. A number of such instances recently have been observed the source of which was never determined.

See previous blog entries [with additional links] as germane:

https://militaryanalysis.blogspot.com/2017/10/radioactive.html

https://militaryanalysis.blogspot.com/2017/11/ru-106.html

https://militaryanalysis.blogspot.com/2018/01/spike.html

https://militaryanalysis.blogspot.com/2018/02/u-235.html

More to follow on this subject. 

 coolbert.