Thorium fuel cycle
A nuclear fuel cycle that uses fertile thorium-232, which absorbs a neutron and decays to fissile uranium-233, as an alternative to uranium-plutonium fuel.
Thorium is about three to four times more abundant than uranium in the Earth's crust and occurs mainly as Th-232 in monazite sands, with large resources in India, Brazil, Australia, Türkiye and the United States. It is not fissile, so it must be bred:
U-233 yields about 2.3 neutrons per thermal neutron absorbed, more than U-235 or Pu-239, which makes breeding possible even in thermal reactors. A starter fissile load (LEU or plutonium) is needed to begin the cycle.
Claimed advantages: far less plutonium and minor actinide production, high melting point and chemical stability of thorium dioxide fuel, and potential for high burnup. Drawbacks: U-232 contamination produces strong gamma emitters that complicate fuel fabrication; Pa-233 must be managed to avoid neutron losses; reprocessing (the THOREX process) is less developed; and separated U-233 is weapons-usable.
History includes the Shippingport reactor's light-water breeder core (1977–1982), German and US high-temperature reactors, and the Molten-Salt Reactor Experiment at Oak Ridge (1965–1969). India's three-stage programme aims to use its thorium reserves in its Advanced Heavy Water Reactor. China started a 2 MWth thorium molten salt reactor in Wuwei in 2023, which was reported refuelled at power in 2025.
Related terms
All terms →- Nuclear fission
The splitting of a heavy atomic nucleus such as uranium-235 or plutonium-239 into lighter nuclei after absorbing a neutron, releasing about 200 MeV of energy and further neutrons.
- Uranium enrichment
The process of increasing the share of fissile U-235 in uranium above its natural 0.711%, to make reactor fuel or, at very high levels, weapons material.
- Burnup
The thermal energy extracted from nuclear fuel per unit mass of initial heavy metal, usually expressed in gigawatt-days per tonne (GWd/tHM).
- Spent nuclear fuel
Fuel assemblies removed from a reactor after use, highly radioactive and thermally hot, containing uranium, plutonium, fission products and minor actinides.
- SMR (Small Modular Reactor)
A nuclear reactor of up to about 300 MWe per module, designed for factory fabrication and modular deployment to cut construction time and financing risk.