Thorium
About
Thorium is a naturally-occurring radioactive element named for the Norse god Thor. Thorium has a 14 billion year half-life, and is almost as abundant as lead. Along with uranium decay, the radioactive decay of thorium is the primary source of thermal heat in the Earth's mantle. One non-nuclear use of thorium is as an impurity in high-performance camera lenses.
Although thorium shows promise as a nuclear fuel, its use in nuclear reactors is 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.
Thorium fuel cycle
Thorium has one naturally-occurring isotope, thorium-232. Thorium itself will not fission upon exposure to neutron radiation like uranium-235, but it will capture a neutron and quickly undergo beta decay (emit an electron) to become protactinium-233. Protactinium-233 has a half-life of about 27 days, and it decays into uranium-233, which is fissile (likely to undergo fission upon exposure to neutrons). Because it can be used to create or "breed" nuclear fuel, thorium is considered fertile.
If protactinium-233 is left exposed to neutron radiation, uranium-232 can be formed. Uranium-232, along with its decay products, emit high-energy gamma radiation. This property can be used to discourage uranium-233 proliferation and weaponization, by explicitly designing reactors to only be fueled with thorium, and to make the extraction of protactinium difficult.
Advantages
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.
Disadvantages
Like any fissile material, uranium-233 can be weaponized. One such 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.
Links
Reference
- Wikipedia
- Conservapedia
: two sentences as of 2013-01-27; no mention of Thorium nuclear power
