Thorium-232

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 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

Source:
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. This 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. There are no minor branches in this chain, and it proceeds as shown:
Or the same in tabular form:

Nuclide	Decay mode	Half-life	Energy released MeV	Decay product
²³²Th	α	1.40 a	4.082	²²⁸Ra
²²⁸Ra	β⁻	5.75 a	0.046	²²⁸Ac
²²⁸Ac	β⁻	6.15 h	2.123	²²⁸Th
²²⁸Th	α	1.9125 a	5.520	²²⁴Ra
²²⁴Ra	α	3.632 d	5.789	²²⁰Rn
²²⁰Rn	α	55.6 s	6.405	²¹⁶Po
²¹⁶Po	α	143.7 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	4.999	²⁰⁸Pb
²⁰⁸Pb	stable

Rare decay modes

Although thorium-232 mainly alpha-decays, it also undergoes spontaneous fission 1.1% of the time, for a partial half-life of 1.3 years, the longest known for that mode. Double beta decay to uranium-232 is also theoretically possible, but has not been observed.

Use in nuclear power

Thorium-232 is not fissile; it therefore cannot be used directly as fuel in nuclear reactors. 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|.
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.