As for its radioactivity, the half-life of thorium (the time it would take for 1/2 half of it to radioactively decay away into something else) is extremely long, hence the radioactivity is very low. A brick house will give off a lot more radioactivity (in total) than one of those mantles from the trace amounts of uranium in the originating material (i.e., clay).

An old watch with radium dials gives off a tremendous amount of radiation, by comparison. And radium mixed with beryllium gives off neutrons, which when slowed down (by water or paraffin) will interact with other elements (like thorium) and make them radioactive through their atoms being split. That's what that kid did, in essence.

"Thorium dioxide is also much more resistant to chemical interactions, which is demonstrated by its uses as a high temperature crucible (container) or in the gas mantle (used in "Coleman" camping laterns) invented in 1885 by Welsbach; which is a fabric soaked with a mixture of thorium nitrate and cerium nitrate, which upon ignition becomes a 99% Thoria "mantle" that glows white-hot when heated by a gas flame."

http://www.easylivingsystems.com/gas-litemantles.htm

"Dr. Carl Auer von Welsbach was an Austrian chemist and engineer who invented the gas mantle,
thus allowing the greatly increased output of light by gas lamps.
In 1885 Welsbach discovered and isolated the elements neodymium and praseodymium from a mixture called didymium,
which was previously considered an element. His interest in rare-earth elements continued,
and he found that a fabric impregnated with a mixture of thorium nitrate and cerium nitrate
could be made into a mantle that glowed brightly when heated by a gas flame.
Patented in 1885, the Welsbach mantle greatly improved gas lighting and,
although largely supplanted by the incandescent lamp, is still widely used in kerosene and other lanterns.
In 1898 Welsbach introduced the first metallic filament for incandescent lamps.
Although the osmium he used was too rare for general use, his improvement paved the way for the tungsten filament
and the modern light bulb. Welsbach also developed misch metal,
a mixture of cerium and other rare earths, which he combined with iron to make Auer's metal,
the first improvement over flint and steel for making sparks since ancient times.
It is used in cigarette lighters and in strikers for lighting gas jets. "

http://www.rottnest.freeserve.co.uk/links.htm

Unfortunately, even gas illumination has environmental implications:
http://www.epa.gov/region02/superfund/npl/0203580c.htm
http://www.epa.gov/superfund/sites/rodsites/0203580.htm

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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(sorry, fairly long...)

The do-it- yourself reactor
David Hahn wanted to be an Eagle Scout, but he was not interested in typical merit-badge projects. Instead of astronomy, backpacking, or business, Hahn was interested in producing energy. He set out to build his very own breeder reactor in his mother's potting shed.

Hahn never built anything resembling a working reactor. But his merit-badge project got some attention. On June 25, 1995 agents from the Environmental Protection Agency (EPA) raided his home in the quiet Detroit suburb of Golf Manor. The extremely high levels of radioactivity the EPA found in his backyard shed prompted the agency to declare it a federal Superfund site.

Hahn was remarkably resourceful in his search for radioactive substances. According to Ken Silverstein ("The Radioactive Boy Scout," Harper's Magazine, November 1998), Hahn "figured out a way to dupe officials at the Nuclear Regulatory Commission [NRC into providing him with crucial information he needed in his attempt to build a breeder reactor, and then he obtained and purified radioactive elements such as radium and thorium."

Even at age 15, Hahn knew how to ask nrc officials for information about how to isolate radioactive elements, which isotopes were capable of sustaining a chain reaction, and where he could find commercial sources of radioactive materials. "The nrc gave me all the information I needed," he told Silverstein. "All I had to do was go out and get the materials."

Not that he had to look very far for most of his "reactor" elements. In an interview on cbs This Morning (October 19, 1998), Hahn listed his sources: "I found lantern mantels, which contain thorium dioxide. They sold these at Kmarts. . . . [I got] tritium from bow-and-arrow sights, polonium from electrostatic film brushes, americium from smoke detectors, radium from, of course, radium dials."

But, writes Silverstein, what Hahn really wanted was uranium 235, because it would provide the "biggest reaction." His first approach was to drive hundreds of miles through northern Michigan with a Geiger counter on his dashboard. The search yielded only a few rocks of pitchblende (which contains minuscule amounts of uranium 235), so he decided to go back to his friends at the nrc for help. His contacts at the regulatory commission gave him the address of a Czechoslovakian firm that sells uranium to commercial buyers. That contact resulted in a few more samples of pitchblende. His attempts to isolate and purify the uranium, however, resulted in little more than a small pile of black powder.

Armed with his radioactive cocktail––and a sketch of a checkerboard breeder reactor from one of his father's college textbooks–– Hahn began devising his backyard breeder. He knew that it took at least 30 pounds of enriched uranium––vastly beyond anything he had––to initiate a chain reaction, but he thought he could at least generate some interesting results. "No matter what happened," he told Silverstein, "there would be something changing into something––some kind of reaction going on there."

His makeshift reactor had a "core" of radium, americium, and beryllium that was wrapped in a "blanket" of thorium ash and uranium powder. And it produced. "The level of radiation after a few weeks was far greater than it was at assembly," he recounted. "I know I transformed some radioactive materials."

When his Geiger counter began picking up radiation several doors down from his house, Hahn decided that he had "too much radioactive stuff in one place," and he disassembled the "reactor." By the time the EPA showed up, he had hidden most of the radioactive materials, including the purified thorium pellets, leaving only the radium and americium behind. "The funny thing is," he joked to Silverstein, "they only got the garbage, and the garbage got all the good stuff."

The "garbage" was enough to scare the EPA, which spent $60,000 on the Golf Manor Superfund clean-up. And, according to CBS This Morning, the agency "never sent a bill."
––Michael Flynn