Now 17, David hit on the idea of building a model breeder reactor, a
nuclear reactor that not only generates electricity, but also produces new
fuel. His model would use the actual radioactive elements and produce real
reactions. His blueprint was a schematic in one of his father’s textbooks.
Ignoring safety, David mixed his radium and americium with beryllium and
aluminum, all of which he wrapped in aluminum foil, forming a makeshift
reactor core. He surrounded this radioactive ball with a blanket of small
foil-wrapped cubes of thorium ash and uranium powder, tenuously held
together with duct tape.
From “TALE OF THE RADIOACTIVE BOY SCOUT,” found via a comment by Tom Holsinger on the Radiation Surveillance thread on Volokh Conspiracy. It turns out that the article has been turned into a book (The Radioactive Boy Scout). Picture from this site.