Physics:Thorium-232

Thorium-232, ²³²Th

General
Symbol	232Th
Names	thorium-232, Th-232
Protons	90
Neutrons	142
Nuclide data
Natural abundance	99.98%±0.02%
Half-life	1.40×1010 years
Parent isotopes	236U (α) 232Ac (β−)
Decay products	228Ra
Isotope mass	232.0380536 u
Spin	0+
Decay modes
Decay mode	Decay energy (MeV)
alpha decay	4.082
Isotopes of Chemistry:thorium Complete table of nuclides

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:

$\begin{array}{l}\\ {\phantom{A}}_{\phantom{90}}^{\phantom{232}}{\phantom{A}}_{\phantom{2}90}^{\phantom{2}232}\mathrm{T}\mathrm{h}\underset{1.40\times 10{\phantom{A}}^{10}\mathrm{y}}{\overset{\mathrm{\alpha }}{\to }}{}^{\mathrm{2}\mathrm{2}\mathrm{8}}{}_{\mathrm{8}\mathrm{8}}\mathrm{R}\mathrm{a}\underset{5.75\mathrm{y}}{\overset{\mathrm{\beta }{\phantom{A}}^{-}}{\to }}{}^{\mathrm{2}\mathrm{2}\mathrm{8}}{}_{\mathrm{8}\mathrm{9}}\mathrm{A}\mathrm{c}\underset{6.15\mathrm{h}}{\overset{\mathrm{\beta }{\phantom{A}}^{-}}{\to }}{}^{\mathrm{2}\mathrm{2}\mathrm{8}}{}_{\mathrm{9}\mathrm{0}}\mathrm{T}\mathrm{h}⟶[\mathrm{\alpha }][1.9125\mathrm{y}]{}^{\mathrm{2}\mathrm{2}\mathrm{4}}{}_{\mathrm{8}\mathrm{8}}\mathrm{R}\mathrm{a}⟶[\mathrm{\alpha }][3.632\mathrm{d}]{}^{\mathrm{2}\mathrm{2}\mathrm{0}}{}_{\mathrm{8}\mathrm{6}}\mathrm{R}\mathrm{n}\\ {\phantom{A}}_{\phantom{86}}^{\phantom{220}}{\phantom{A}}_{\phantom{2}86}^{\phantom{2}220}\mathrm{R}\mathrm{n}\underset{55.6\mathrm{s}}{\overset{\mathrm{\alpha }}{\to }}{}^{\mathrm{2}\mathrm{1}\mathrm{6}}{}_{\mathrm{8}\mathrm{4}}\mathrm{P}\mathrm{o}\underset{144.0\mathrm{m}\mathrm{s}}{\overset{\mathrm{\alpha }}{\to }}{}^{\mathrm{2}\mathrm{1}\mathrm{2}}{}_{\mathrm{8}\mathrm{2}}\mathrm{P}\mathrm{b}\underset{10.627\mathrm{h}}{\overset{\mathrm{\beta }{\phantom{A}}^{-}}{\to }}{}^{\mathrm{2}\mathrm{1}\mathrm{2}}{}_{\mathrm{8}\mathrm{3}}\mathrm{B}\mathrm{i}\left\{\begin{array}{c}\underset{60.55\mathrm{m}\mathrm{i}\mathrm{n}}{\overset{64.06\mathrm{\%}\mathrm{\beta }{\phantom{A}}^{-}}{\to }}{}^{\mathrm{2}\mathrm{1}\mathrm{2}}{}_{\mathrm{8}\mathrm{4}}\mathrm{P}\mathrm{o}\underset{294.4\mathrm{n}\mathrm{s}}{\overset{\mathrm{\alpha }}{\to }}\\ \underset{60.55\mathrm{m}\mathrm{i}\mathrm{n}}{\overset{35.94\mathrm{\%}\mathrm{\alpha }}{\to }}{}^{\mathrm{2}\mathrm{0}\mathrm{8}}{}_{\mathrm{8}\mathrm{1}}\mathrm{T}\mathrm{l}\underset{3.053\mathrm{m}\mathrm{i}\mathrm{n}}{\overset{\mathrm{\beta }{\phantom{A}}^{-}}{\to }}\end{array}\right\}{\phantom{A}}_{\phantom{82}}^{\phantom{208}}{\phantom{A}}_{\phantom{2}82}^{\phantom{2}208}\mathrm{P}\mathrm{b}\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.

Nuclide	Decay mode	Half-life (y = years)	Decay Q-value (MeV)	Decay product
232Th	α	1.40×1010 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 1.1×10⁻⁹% of thorium-232 decays, corresponding to a partial half-life of about 1.3×10²¹ years. 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

Main articles: Physics:Thorium-based nuclear power and Thorium fuel cycle

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 [[uranium-233|]].

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.

References

Lighter: thorium-231	Thorium-232 is an isotope of thorium	Heavier: thorium-233
Decay product of: uranium-236 actinium-232	Decay chain of thorium-232	Decays to: radium-228