Thorium-232

Thorium-232 (232Th) 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:

Th 90 232 → 1.40 × 10 10 y α Ra 88 228 → 5.75 y β − Ac 89 228 → 6.15 h β − Th 90 228 → 1.9125 y α Ra 88 224 → 3.632 d α Rn 86 220 Rn 86 220 → 55.6 s α Po 84 216 → 144.0 m s α Pb 82 212 → 10.627 h β − Bi 83 212 { → 60.55 min 64.06 % β − Po 84 212 → 294.4 ns α → 60.55 min 35.94 % α Tl 81 208 → 3.053 min β − } Pb 82 208 {\displaystyle {\begin{array}{l}{}\\{\ce {^{232}_{90}Th->[\alpha ][1.40\times 10^{10}\ {\ce {y}}]{^{228}_{88}Ra}->[\beta ^{-}][5.75\ {\ce {y}}]{^{228}_{89}Ac}->[\beta ^{-}][6.15\ {\ce {h}}]{^{228}_{90}Th}->[\alpha ][1.9125\ {\ce {y}}]{^{224}_{88}Ra}->[\alpha ][3.632\ {\ce {d}}]{^{220}_{86}Rn}}}\\{\ce {^{220}_{86}Rn->[\alpha ][55.6\ {\ce {s}}]{^{216}_{84}Po}->[\alpha ][144.0\ {\ce {m}}s]{^{212}_{82}Pb}->[\beta ^{-}][10.627\ {\ce {h}}]{^{212}_{83}Bi}}}{\begin{Bmatrix}{\ce {->[64.06\%\beta ^{-}][60.55\ {\ce {min}}]{^{212}_{84}Po}->[\alpha ][294.4\ {\ce {ns}}]}}\\{\ce {->[35.94\%\alpha ][60.55\ {\ce {min}}]{^{208}_{81}Tl}->[\beta ^{-}][3.053\ {\ce {min}}]}}\end{Bmatrix}}{\ce {^{208}_{82}Pb}}\end{array}}}

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.

Rare decay modes

Spontaneous fission accounts for about 1.1×10−9% of thorium-232 decays, corresponding to a partial half-life of about 1.3×1021years. 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, 232Th is fertile: it can capture a neutron to form 233Th, which undergoes a beta decay with a half-life of 21.8 minutes to 233Pa, then another with a half-life of 27 days to form fissile 233U.

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.