Thorium is a silvery-white metal that occurs naturally in the earth’s crust and can be grown in a nuclear reactor to the fissile isotope U-233. It has potential as a nuclear fuel source and is used as an alloying element with other metals. Thorium is radioactive but has a long half-life. It is not widely available due to safety concerns, but some governments have developed plans to build thorium-fired reactors in the event of a uranium supply disruption. Thorium mantles were frequently used as a light source before electric lighting.
Thorium (symbol Th, atomic number 90) is a radioactive chemical element. It is a silvery-white metal at room temperature, but oxidizes easily when exposed to air and occurs naturally only in oxidized form. Although thorium is not fissionable, it can be grown in a nuclear reactor to the fissile isotope U-233, and therefore has potential as a nuclear fuel source. It is also used as an alloying element with other metals and is the main ingredient in gas lantern mantles.
This element occurs naturally in the earth’s crust, at a concentration of about 12 ppm (about the same as lead, and three times that of uranium). Although thorium is radioactive, its 14 billion year half-life is so long that most of that originally found on Earth is still there. The primary ore for thorium is the ore monazite, which can have up to 10% by mass; some other minerals, such as thorianite and euxenite, also contain significant amounts.
Although many countries have large reserves, thorium is not widely available; its applications as a metal are limited by its radioactivity, making it potentially dangerous if inhaled or ingested. Thorotrast, a compound once used for medical X-rays, has been abandoned due to safety concerns. Ironically, thorium’s high density and atomic number make it an effective radiation shield, although lead and depleted uranium are used more frequently.
Thorium is not fissile, so it cannot be used to build an atomic bomb or a nuclear reactor. However, when this element is placed in a nuclear reactor, the high neutron flux causes some of it to transmute into U-233, which is fissile. U-233 can then be used to support the nuclear reaction and transmute more thorium, creating a closed nuclear fuel cycle, making it potentially valuable as an energy source. Historically, natural uranium has been cheap enough as a fuel that thorium is redundant. However, with uranium prices soaring, some governments have developed plans to build thorium-fired reactors in the event of a uranium supply disruption; some heavy water reactors, such as the CANDU design, can already use the element.
Before the advent of electric lighting, thorium mantles were frequently used as a light source; when heated with a flame, some thorium dioxide alloys will emit a dazzling white light. This glow is unrelated to radioactivity and results from chemical interactions with cerium and oxygen. Unless ingested or otherwise introduced into the body, thorium capes and other products are generally safe enough for everyday use, as the alpha particles given off by the element cannot penetrate the skin.
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