Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half-life of 14.0 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay chain terminates at stable lead-208.
Thorium-232 is a fertile material; it can capture a neutron to form thorium-233, which subsequently undergoes two successive beta decays to uranium-233, which is fissile. As such, it has been used in the thorium fuel cycle in nuclear reactors; various prototype thorium-fueled reactors have been designed. However, as of 2024, thorium fuel has not been widely adopted for commercial-scale nuclear power.
Natural occurrence
The half-life of thorium-232 (14 billion years) is more than three times the age of the Earth; thorium-232 therefore occurs in nature as a primordial nuclide. Other thorium isotopes occur in nature in much smaller quantities as intermediate products in the decay chains of uranium-238, uranium-235, and thorium-232.
Some minerals that contain thorium include apatite, sphene, zircon, allanite, monazite, pyrochlore, thorite, and xenotime.
Decay
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One potential advantage of a thorium-based nuclear fuel cycle is that thorium is three times more abundant than uranium, the current fuel for commercial nuclear reactors. It is also more difficult to produce material suitable for nuclear weapons from the thorium fuel cycle compared to the uranium fuel cycle. Some proposed designs for thorium-fueled nuclear reactors include the molten salt reactor and a fast neutron reactor, among others. Although thorium-based nuclear reactors have been proposed since the 1960s and several prototype reactors have been built, there has been relatively little research on the thorium fuel cycle compared to the more established uranium fuel cycle; thorium-based nuclear power has not seen large-scale commercial use as of 2024. Nevertheless, some countries such as India have actively pursued thorium-based nuclear power.