Thorium fuel cycle
The thorium fuel cycle is a nuclear fuel cycle, or a process by which thorium is expended to produce energy by nuclear fission. Because thorium by itself cannot undergo fission and must be transmuted to a fissile material (uranium-233), it is a fertile material, akin to uranium-238 in the plutonium fuel cycle.
Thorium has one natural isotope: thorium-232. When exposed to neutrons, the thorium nucleus can capture a neutron and quickly beta decay (emit an electron) to become protactinium-233. Protactinium-233 has a half-life of approximately 27 days, and beta decays to fissile uranium-233. When uranium-233 undergoes fission, the reaction emits 2 or 3 neutrons, which can be captured by other thorium nuclides, or fission other uranium-233 nuclides, and thus propagate the fuel cycle.
If protactinium-233 remains exposed to neutrons, it can be converted to protactinium-232, which beta decays to uranium-232. Uranium-232 and many of its decay products pose a gamma radiation hazard even at concentrations of 5 ppm. While this makes handling uranium-233 potentially difficult, it also means that allowing uranium-232 to form can be an effective proliferation deterrent in reactors that use thorium. Uranium-232 can slow a nuclear chain reaction as it absorbs neutrons (hampering attempts to make uranium-233 supercritical), and its gamma radiation can severely damage any electrical equipment or explosive materials used in weapons, not to mention prospective weapon makers without adequate shielding.
It is possible to produce uranium-233 of high purity, in a once-through process that irradiates thorium and immediately separates out protactinium. Other than the research reactors specifically designed to produce uranium-233 in the US nuclear program, very few thorium reactor designs immediately extract protactinium-233.