Discovered in 1828 by Sweden-based chemist Jons Jakob Berzelius, thorium is a naturally-occurring radioactive material found in small amounts in rocks and soils. It is about three times more abundant than uranium. Thorium exists in a single isotopic form – Th-232 – that decays slowly. Monazite, a rare earth phosphate mineral, is the common source of thorium. It contains up to 12% thorium phosphate. Monazite is found in igneous and other rocks with richest deposits in areas concentrated by waves and current action with other heavy minerals. World monazite resources are estimated to stand at about 16 million tons of which 12 million tons are in heavy mineral sands deposits on the south and east coasts of India.
Thorium is not fissile; therefore, it is not directly usable in a thermal neutron reactor. However, it is ‘fertile’ and upon absorbing a neutron transmutes to uranium-233 (U-233), which is an excellent fissile fuel material. All thorium fuel concepts therefore require that Th-232 first be irradiated in a reactor to provide the necessary neutron dosing to produce protactinium-233. The Pa-233 that is produced can either be chemically separated from the parent thorium fuel and the decay product U-233 then recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form, especially in molten salt reactors (MSRs). Therefore, thorium fuels need a fissile material as a ‘driver’ so that a chain reaction can be maintained. The only fissile driver options are U-233, U-235, or Pu-239, these are not easily available.
A basic design principle in thorium fuel systems is that of heterogeneous fuel arrangement, wherein a high fissile fuel zone called the seed region is physically separated from the fertile thorium part of the fuel known as blanket. Such an arrangement is better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U-233. This principle applies to all the thorium-capable reactor systems. There are seven types of reactors into which thorium can be introduced as a nuclear fuel: Heavy Water Reactors (PHWRs), High-Temperature Gas-Cooled Reactors (HTRs), Boiling (Light) Water Reactors (BWRs), Pressurized (Light) Water Reactors (PWRs), Fast Neutron Reactors (FNRs), Molten Salt Reactors (MSRs), and Accelerator Driven Reactors (ADS).
A thorium fuel cycle offers several potential advantages over uranium fuel cycle. These include much greater abundance on earth, superior physical and nuclear fuel properties, and reduced nuclear waste production. Since 2008, nuclear energy experts have shown key interests in thorium to supply nuclear fuel in place of uranium in order to generate nuclear power. Thorium is key in developing a new generation of cleaner and safer nuclear power. Considering its overall potential, thorium-based power can mean a 1000+ year solution or a quality low-carbon bridge towards sustainable energy sources. This would help eradicate a large portion of the mankind’s negative environmental impact.
The major reactor manufacturing companies are General Electric, Mitsubshi Heavy Industries, Terrestrial Energy, Moltex Energy, ThorCon Power, Terra Power, Flibe Energy, Transatomic Power Corporation, Thor Energy