AdvertisementClues In That Mysterious Radioactive Cloud Point Toward RussiaNovember 17, 2017Geoff BrumfielAlina SelyukhfacebookEmailThe core of the RBT-3 reactor at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. Some scientists suspect the institute's work on medical isotopes might explain radioactivity detected over Europe. (UIG via Getty Images)