Thorium is fertile, not fissile
A reactor fuel must do more than contain atoms that can split. To sustain a chain reaction, its nuclei must split when struck by neutrons at a rate that keeps enough new neutrons available to cause further splits. Thorium-232 is fertile: it can be transformed into a fissile fuel, but it does not readily start that multiplying cycle itself.
The neutron starts a conversion chain
When thorium-232 captures a neutron, it becomes thorium-233. That unstable nucleus changes through radioactive decay—first into protactinium-233, then into uranium-233. Uranium-233 is fissile: a neutron can split one of its nuclei, releasing energy and additional neutrons.
Those newly released neutrons can split more uranium-233, while some are captured by other thorium atoms. In this way, a reactor can both generate power and make more of the material that sustains fission. The two processes must be balanced: too many neutrons escape or get absorbed in the wrong places, and the chain reaction cannot continue.
A starter fuel—and careful neutron management
Because thorium cannot supply the initial multiplying reaction, a thorium-fueled system needs a source of neutrons, typically from a fissile fuel such as uranium-235 or plutonium. That fuel helps start and maintain fission while thorium is converted into uranium-233. Reactor design determines how effectively the available neutrons serve both purposes.
The key distinction is that thorium is not a ready-made substitute that can simply be loaded and switched on. It is a fuel-making material: the reactor must first provide the neutron-driven steps that turn it into fissile uranium.
