What is Thorium?
Thorium is a slightly radioactive metal with small ammounts naturally being found in small amounts in most rocks and soils. It is three times more abundant than uranium. Within soil, there is an average of 6 parts per million of thorium. Thorium is insoluble and unlike uranium, is plentiful in sands but not in seawater. Thorium is a single isotope, Th-232, which decays very slowly. It has a half-life of about three times the age of the Earth.
What does Thorium look like?
Thorium is a silvery white metal that retains its lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. When heated in air, thorium metal ignites and burns brilliantly with a white light.
What do we use Thorium for?
Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C) and so it has found applications in light bulb elements, lantern mantles, arc-light lamps, welding electrodes and heat-resistant ceramics. Glass containing thorium oxide has both a high refractive index and wavelength dispersion, and is used in high quality lenses for cameras and scientific instruments.
How much Thorium is there?
The most common source of thorium is the rare earth phosphate mineral, monazite, which contains up to about 12% thorium phosphate. World monazite resources are estimated to be about 16 million tonnes.Thorite (ThSiO4) is another common thorium mineral. A large vein deposit of thorium and rare earth metals is in Idaho,United States.
How can we use Thorium as an energy source?
Thorium (Th-232) is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 which is an excellent fissile fuel material similar to uranium-238 which transmutes to plutonium-239. All thorium fuel concepts require the Th-232 is first 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).
Using thorium as a fuel:
Another option for using thorium as a fuel is a ‘fertile matrix’ for fuels containing plutonium that serves as the fissile driver while being consumed (and even other transuranic elements like americium. Mixed thorium-plutonium oxide (Th-Pu MOX) fuel is an analog of current uranium-MOX fuel, but no new plutonium is produced from the thorium component, unlike for uranium fuels in U-Pu MOX fuel, and so the level of net consumption of plutonium is high. Production of all actinides is lower than with conventional fuel, and negative reactivity coefficient is enhanced compared with U-Pu MOX fuel. In fresh thorium fuel, all of the fissions (thus power and neutrons) derive from the driver component. As the fuel operates the U-233 content gradually increases and it contributes more and more to the power output of the fuel. The ultimate energy output from U-233 (and hence indirectly thorium) depends on numerous fuel design parameters, including: fuel burn-up attained, fuel arrangement, neutron energy spectrum and neutron flux (affecting the intermediate product protactinium-233, which is a neutron absorber). The fission of a U-233 nucleus releases about the same amount of energy (200 MeV) as that of U-235.
An important principle in the design of thorium fuel systems is that of heterogeneous fuel arrangement in which a high fissile (and therefore higher power) fuel zone called the seed region is physically separated from the fertile (low or zero power) thorium part of the fuel – often called the blanket. Such an arrangement is far better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U-233, in fact all thermal breeding fuel designs are heterogeneous. This principle applies to all the thorium-capable reactor systems.
What type of reactors are able to use Thorium?
- Heavy Water Reactors (PHWRs)
- High-Temperature Gas-Cooled Reactors (HTRs)
- Accelerator Driven Reactors (ADS)
- Molten Salt Reactors (MSRs)
- Fast Neutron Reactors (FNRs)
- Pressurised (Light) Water Reactors (PWRs)
- Boiling (Light) Water Reactors (BWRs)
- Thorium is a cleaner, safer, and more abundant nuclear fuel that has the potential to revolutionize energy production.
- Thorium is more abundant in nature than uranium.
- It is fertile rather than fissile, and can only be used as a fuel in conjunction with a fissile material such as recycled plutonium.
- Thorium fuels can breed fissile uranium-233 to be used in various kinds of nuclear reactors.
- Several significant demonstrations of the use of thorium-based fuels to generate electricity in several reactor types have been displayed in early trials.
- Molten salt reactors are well suited to thorium fuel, as normal fuel fabrication is avoided.
- A thorium fuelled reactor operated from 1977 to 1982 at Shippingport in the USA.
- The use of thorium as a new primary energy source can be a cost effective, clean fuel due to its latent energy value.
- Norway’s Thor Energy is developing and testing two thorium-bearing fuels for use in existing nuclear power plants.
- In India, some heavy water reactors have been used thorium-bearing fuel bundles.
- Several North America and Europe utilities are initiating feasibility studies to investigate the use of Thorium as a fuel source.
- The thorium-fuelled MSR is sometimes referred to as the Liquid Fluoride Thorium Reactor which has been bred in a liquid thorium salt blanket.