Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of approximately 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. In 2025, China reported thorium-to-uranium conversion in an experimental molten-salt reactor.
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 Thorium-232 has a half-life of 14 billion years; it is itself an essentially pure alpha emitter with its first decay product radium-228. Radium-228 is itself unstable and leads to a decay chain known as the thorium series, which terminates at stable lead-208. The intermediates in the thorium-232 decay chain are all relatively short-lived; the longest-lived intermediate decay products are radium-228 and thorium-228, with half-lives of 5.75 years and 1.91 years, respectively. All others have half-lives under four days. The main decay branches are shown below:
The table shows the principal decay paths, with half-lives from NUBASE2020. The energy column gives ground-state-to-ground-state Q-values from AME2020, displayed in NuDat and rounded to 0.001 MeV. These values describe total decay energy, rather than individual particle energies.
NuclideDecay modeHalf-life(y = years)Decay Q-value(MeV)Decayproduct 232Th α 1.40 y 4.082 228Ra 228Ra β− 5.75 y 0.046 228Ac 228Ac β− 6.15 h 2.124 228Th 228Th α 1.9125 y 5.520 224Ra 224Ra α 3.632 d 5.789 220Rn 220Rn α 55.6 s 6.405 216Po 216Po α 144.0 ms 6.906 212Pb 212Pb β− 10.627 h 0.569 212Bi 212Bi β− 64.06% α 35.94% 60.55 min 2.252 6.207 212Po 208Tl 212Po α 294.4 ns 8.954 208Pb 208Tl β− 3.053 min 4.998 208Pb 208Pb stable
Rare decay modes Spontaneous fission accounts for about % of thorium-232 decays, corresponding to a partial half-life of about . Double beta decay to uranium-232 is also theoretically possible. A 2020 experimental search set lower half-life limits for such decays to excited states of uranium-232.
Use in nuclear power
Thorium-232 is not fissile, but it can undergo fission with high-energy neutrons. Its use in a reactor fuel cycle requires a fissile driver, such as uranium or plutonium, to sustain a chain reaction. However, is fertile: it can capture a neutron to form , which undergoes a beta decay with a half-life of 21.8 minutes to , then another with a half-life of 27 days to form fissile .
Thorium is estimated to be about three to four times as abundant as uranium in Earth's upper crust. Uranium-233 produced in thorium fuel cycles raises proliferation concerns. Radiation from uranium-232 decay products can complicate fuel handling. The risks depend on fuel-cycle design and safeguards. A 1958 report described the Indian Point reactor under construction, with thorium as a fertile material to supplement uranium-235 fuel. The Shippingport light-water breeder reactor core operated with fuel containing uranium-233 and thorium. 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.
In November 2025, the Chinese Academy of Sciences reported thorium-to-uranium conversion following thorium loading in an experimental molten-salt reactor built by its Shanghai Institute of Applied Physics and partner institutions. The institute stated that it planned a 100-megawatt demonstration project by 2035.
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