Despite the fact the use of thorium as a potential fuel for nuclear power was known by the 1960s (see Molten-Salt Reactor Experiment [W]), and despite the much greater hazards inherent in uranium fuel, as yet there are no thorium nuclear power plants currently in operation.
The story is that during the cold war, when the United States was first investigating nuclear power, they had a choice between thorium and uranium. Uranium produces plutonium, which is highly toxic but can be used for nuclear weapons, as a byproduct. This concern apparently trumped both safety and efficiency, so the US put all their development resources into uranium and plutonium-fueled nuclear, leading to the nuclear power plants we have today. Also, the political support was much stronger for the liquid metal fast breeder reactor and the plutonium fuel cycle, so research into thorium was effectively killed when Dr. Alvin Weinberg was fired from Oak Ridge National Laboratory.
There are, however, a number of projects are in the testing and development phases. India's plan to use thorium in nuclear power is the best-known, and as of 2012, China is researching thorium for use in generation IV reactor designs.
The thorium fuel cycle [W] was discovered by Glenn Seaborg [W][?]. Briefly, when thorium is irradiated by neutrons, it is transmuted to protactinium-233, which decays into uranium-233, which can then be used in a fission reactor to produce energy.
- molten salt reactor (MSR), a generation IV thermal spectrum (slow neutron) nuclear reactor design for its safety and efficiency
- liquid fluoride thorium reactor (LFTR), a variant of the MSR currently being prototyped by Flibe Energy [W]
- As of 2010, a Virginia-based company called Lightbridge has designed a nuclear fuel assembly designed for existing reactors of a particular type ("light water") but using thorium as its primary fuel.
- In January 2011, the Chinese Academy of Sciences formally announced a research project on thorium-fueled molten salt reactors.
- In April 2011, thorium advocate and former NASA engineer Kirk Sorensen founded Flibe Energy [W] to design and construct a variant of the MSR called liquid fluoride thorium reactor, or LFTR.
- In India, thorium nuclear power is part of the nation's "three stage nuclear power programme", a long-term research project conceived in the 1950s aimed at both developing advanced nuclear power and exploiting India's large reserves of thorium. Two research reactors that include thorium in their cores are currently in operation, and a prototype stage two breeder reactor which includes a thorium oxide blanket is within a year of operation as of 2012.
Thorium is abundant in the earth's crust, and the fact that all natural thorium can be used to breed nuclear fuel means that no isotopic separation is necessary to use thorium in nuclear reactors. If thorium is used in a reactor that promotes efficient consumption of the fuel like a molten salt reactor, even a small amount can generate large amounts of energy. Because thorium is often found in minerals rich in rare earth elements such as neodymium, if thorium becomes recognized as a precursor to useful nuclear fuel, it can be an additional revenue source for rare earth miners, rather than a regulatory nuisance requiring special disposal methods due to radioactivity.
The cross-section, or affinity to capture neutrons, of uranium-233 is very large, and has a 92% chance of fissioning upon capturing a slow or thermal spectrum neutron. This is higher than plutonium-239, and even uranium-235, making uranium-233 a very suitable nuclear fuel.
Like any fissile material, uranium-233 can be weaponized. The only known uranium-233-based weapon, used in the Operation Teapot nuclear tests, contained a mixture of fissile plutonium and uranium-233. While it didn't have the power the US military was expecting, the test demonstrated definitively that if very pure uranium-233 can be produced, it can sustain the nuclear chain reaction needed to create a weapon. Uranium-232 contamination, or using natural uranium to dilute or denature the fissile material in storage, would be necessary to discourage proliferation.
If used in conventional solid-fueled reactors, thorium confers few advantages other than abundance and the high melting point of thorium oxide.
- Thorium 101 for Human Beings
- US DOE brief on Dr. Seaborg, including U-233 isolation and discovery of thorium cycle
- 2010-10-20 The Truth About Thorium and Nuclear Power