An alternative to the uranium fuel cycle is the thorium cycle. Key facts:
- Thorium-232 (Th-232) is fertile (not fissile). Under neutron irradiation it is converted to uranium-233 (U-233), which is fissile.
- Thorium is roughly three times more abundant than uranium in the Earth’s crust, with major deposits in India and Brazil (mainly in monazite sands).
- Irradiated thorium fuel is less suitable for weapons proliferation owing to the build-up of U-232, which has a highly radioactive decay chain that makes handling difficult.
- Thorium oxide (ThO₂) has greater chemical stability than uranium oxide.
- For reactors using thorium with high burn-up, a mixed thorium-uranium fuel allows more constant reactivity owing to thorium’s high conversion ratio.
- Thorium fuel has not been extensively used to date because the existing global infrastructure is built around the uranium fuel cycle, and breeder reactors (which are needed to make thorium economically attractive) have not yet been commercially successful.
- India is the country most actively pursuing the thorium cycle, owing to its large thorium reserves and relatively limited uranium deposits.