So far we have focused on uranium and plutonium in our discussion of nuclear fuel and reactors because they are the fuels for most of the world's reactors. There are other nuclear fuels used in existing reactors and atomic batteries or suggested for use in new designs. One of these alternative nuclear fuels that holds great promise is thorium.
Thorium is a naturally occurring radioactive element. It is abbreviated as Th and has ninety protons in its nucleus. Almost all natural thorium is the isotope Th-232. It has a half-life of about fourteen billion years or the estimated current age of the universe. There is about four times as much thorium in the Earth's crust as uranium.
Mining and processing of rare earths yields thorium as a by-product. Pure thorium is a silvery metal that is soft and ductile. Powdered thorium is can ignite and burn when exposed to air. A common use of thorium was as a component in alloys such as a magnesium alloy called Mag-Thor that was used in the aerospace industry for engines because of its stability at high temperatures. It has also been used in electronic applications and welding rods. Thorium dioxide has a very high melting point was used in the mantles of gas lamps and as an additive for high temperature ceramics and laboratory glassware Recent concerns over radioactivity have ultimately made thorium and thorium dioxide unattractive for these application.
Thorium has a long history in the nuclear industry. Thorium is attractive as a nuclear fuel because it is safer and more abundant than uranium. It was used in an experimental reactor built in 1962 and was the fuel chosen in 1977 for a reactor at the first civilian nuclear plant in built in the U.S. at Shippingport Pennsylvania. In the mid 1990s, the US Department of Energy and the International Atomic Energy Commission funded research into thorium as a nuclear fuel. A consortium of major corporations was formed for thorium fuel research as well. India and China have funded major projects to use thorium for power production.
One benefit of thorium fuel is the fact that is it more difficult to extract weapons grade uranium-233 from a thorium reactor that from a uranium reactor. A second benefit is that thorium products much less nuclear waste with long half-lives. A third benefit is that thorium can be used for fuel just as it comes out of the ground without the need to expensive and complex enrichment. And, fourth, thorium cannot sustain a chain reaction without an external source of neutron flux and so, if the external neutron source is deactivated, there is no risk of a core meltdown.
Some of the problems with using thorium for nuclear fuel include the need for research and development in fabricating thorium fuel rods, thorium reactor design, thorium reprocessing and possible production of weapons-grade in special breeder reactors.
Thorium use for nuclear fuel was not attractive as long as there was plenty of cheap uranium. But rising uranium prices, problems with nuclear waste, fear of nuclear weapons production and other problems have made thorium an attractive alternative nuclear fuel in the twenty first century. There is enough thorium inside the United States to provide for our current level of power consumption for over one thousand years.