Thorium-232

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.

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.

[4] Some minerals that contain thorium include apatite, sphene, zircon, allanite, monazite, pyrochlore, thorite, and xenotime.

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.

Ball-and-arrow presentation of the thorium decay series
The 4 n decay chain of 232 Th, commonly called the "thorium series"