THORIUM & ITS SOURCES
Varun Gupta
Thorium
(Updated 11 November 2011)
- Thorium is more abundant in nature than uranium.
- It is fertile rather than fissile, and can be used in conjunction with fissile material as nuclear fuel.
- Thorium fuels can breed fissile uranium-233.
The use of thorium as a new primary energy source has been a tantalizing prospect for many years. Extracting its latent energy value in a cost-effective manner remains a challenge, and will require considerable R&D investment.
Nature and sources of thorium
Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium.
Thorium exists in nature in a single isotopic form - Th-232 - which decays very slowly (its half-life is about three times the age of the Earth). The decay chains of natural thorium and uranium give rise to minute traces of Th-228, Th-230 and Th-234, but the presence of these in mass terms is negligible.
When pure, 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. Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Because of these properties, thorium has found applications in light bulb elements, lantern mantles, arc-light lamps, welding electrodes and heat-resistant ceramics. Glass containing thorium oxide has a high refractive index and dispersion and is used in high quality lenses for cameras and scientific instruments.
The most common source of thorium is the rare earth phosphate mineral, monazite, which contains up to about 12% thorium phosphate, but 6-7% on average. Monazite is found in igneous and other rocks but the richest concentrations are in placer deposits, concentrated by wave and current action with other heavy minerals. World monazite resources are estimated to be about 12 million tonnes, two-thirds of which are in heavy mineral sands deposits on the south and east coasts of India. There are substantial deposits in several other countries (see Table below). Thorium recovery from monazite usually involves leaching with sodium hydroxide at 140°C followed by a complex process to precipitate pure ThO2.
Thorite (ThSiO4) is another common mineral. A large vein deposit of thorium and rare earth metals is in Idaho.
The 2007 IAEA-NEA publication Uranium 2007: Resources, Production and Demand (often referred to as the 'Red Book') gives a figure of 4.4 million tonnes of total known and estimated resources, but this excludes data from much of the world. Data for reasonably assured and inferred resources recoverable at a cost of $80/kg Th or less are given in the table below. Some of the figures are based on assumptions and surrogate data for mineral sands, not direct geological data in the same way as most mineral resources.
Estimated world thorium resources1
| (Reasonably assured and inferred resources recoverable at up to $80/kg Th) | ||
| Country | Tonnes | % of total |
| Australia | 489,000 | 19 |
| USA | 400,000 | 15 |
| Turkey | 344,000 | 13 |
| India | 319,000 | 12 |
| Venezuela | 300,000 | 12 |
| Brazil | 302,000 | 12 |
| Norway | 132,000 | 5 |
| Egypt | 100,000 | 4 |
| Russia | 75,000 | 3 |
| Greenland | 54,000 | 2 |
| Canada | 44,000 | 2 |
| South Africa | 18,000 | 1 |
| Other countries | 33,000 | 1 |
| World total | 2,610,000 |
Thorium as a nuclear fuel
Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor – in this regard it is very similar to uranium-238. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233)a, which is an excellent fissile fuel material b. Thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the necessary neutron dosing. The U-233 that is produced can either be chemically separated from the parent thorium fuel and recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form.
Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain reaction (and thus supply of surplus neutrons) can be maintained. The only fissile driver options are U-233, U-235 or Pu-239 (none of which is easy to supply).
It is possible – but quite difficult – to design thorium fuels that produce more U-233 in thermal reactors than the fissile material they consume (this is referred to as having a fissile conversion ratio of more than 1.0 and is also called breeding). Thermal breeding with thorium is only really possible using U-233 as the fissile driver, and to achieve this the neutron economy in the reactor has to be very good (ie, low neutron loss through escape or parasitic absorption). The possibility to breed fissile material in slow neutron systems is a unique feature for thorium-based fuels and is not possible with uranium fuels.
Another distinct option for using thorium is as a ‘fertile matrix’ for fuels containing plutonium (and even other transuranic elements like americium). No new plutonium is produced from the thorium component, unlike for uranium fuels, and so the level of net consumption of this metal is rather high. 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).
An important principle in the design of thorium fuel is that of heterogeneous fuel arrangements 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 – 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.
Reactors able to use Thorium
There are seven types of reactor into which thorium can be introduced as a nuclear fuel. The first five of these have all entered into operational service at some point. The last two are still conceptual:
- Heavy Water Reactors (PHWRs): These are very well suited for thorium fuels due to their combination of: (i) excellent neutron economy (their low parasitic neutron absorption means more neutrons can be absorbed by thorium to produce useful U-233), (ii) slightly faster average neutron energy which favours conversion to U-233, (iii) flexible on-line refueling capability. Furthermore, heavy water reactors (especially Candu) are well established and widely-deployed commercial technology for which there is extensive licensing experience.
There is potential application to Enhanced Candu 6 and ACR-1000 reactors fueled with 5% plutonium (reactor grade) plus thorium. In the closed fuel cycle, the driver fuel required for starting off is progressively replaced with recycled U-233, so that on reaching equilibrium 80% of the energy comes from thorium. Fissile drive fuel could be LEU, plutonium, or recycled uranium from LWR. Fleets of PHWRs with near-self-sufficient equilibrium thorium fuel cycles could be supported by a few fast breeder reactors to provide plutonium. - High-Temperature Gas-Cooled Reactors (HTRs): These are well suited for thorium-based fuels in the form of robust ‘TRISO’ coated particles of thorium mixed with plutonium or enriched uranium, coated with pyrolytic carbon and silicon carbide layers which retain fission gases. The fuel particles are embedded in a graphite matrix that is very stable at high temperatures. Such fuels can be irradiated for very long periods and thus deeply burn to exploit their original fissile charge. Thorium fuels can be designed for both ‘pebble bed’ and ‘prismatic’ HTR fuel varieties.
- Boiling (Light) Water Reactors (BWRs): BWR fuel assemblies allow for structure & composition options, such as extra moderation and/or half-length fuel rods. This design flexibility means that well-optimized thorium fuels can be created for BWRs, for example, thorium-plutonium fuels that are tailored for ‘burning’ plutonium. BWRs are a well-understood and licensed reactor design.
- Pressurised (Light) Water Reactors (PWRs): Viable thorium fuels can be designed for a PWR, though with less flexibility than for BWRs. Fuel needs to be in heterogeneous arrangements in order to achieve satisfactory fuel burn-up. It is not possible to design thorium-based PWR fuels that convert significant amounts of U-233. Even though PWRs are not the perfect reactor in which to use thorium, they are the industry workhorse and there is a lot of PWR licensing experience. They are a viable early-entry thorium platform.
- Fast Neutron Reactors (FNRs): Thorium can serve as a fuel component for reactors operating with a fast neutron spectrum – in which a wider range of heavy nuclides are fissionable and may potentially drive a thorium fuel. There is, however, no relative advantage in using thorium instead of depleted uranium (DU) as a fertile fuel matrix in these reactor systems due to a higher fast-fission rate for U-238 and the fission contribution from residual U-235 in this material. Also, there is a huge amount of surplus DU available for use when more FNRs are commercially available, so thorium has little or no competitive edge in these systems.
- Molten Salt Reactors (MSRs): These reactors are still at the design stage but will be very well suited for using thorium as a fuel. The unique fluid fuel incorporates thorium and uranium (U-233 and/or U-235) fluorides as part of a salt mixture that melts in the range 400-600ºC, and this liquid serves as both heat transfer fluid and the matrix for the fissioning fuel. The fluid circulates through a core region and then through a chemical processing circuit that removes various fission products (poisons) and/or the valuable U-233. Certain MSR designs [c] will be designed specifically for thorium fuels to produce useful amounts of U-233 – eventually leading to the self-sustaining use of thorium as an energy source.
- Accelerator Driven Reactors (ADS): The sub-critical ADS system is an unconventional concept that is potentially ‘thorium capable’. Spallation neutrons are produced d when high-energy protons from an accelerator strike a heavy target like lead. These neutrons are directed at a region containing a thorium fuel, eg, Th-plutonium which reacts producing heat as in a conventional reactor. The system remains subcritical ie, unable to sustain a chain reaction without the proton beam. Difficulties lie with the reliability of high-energy accelerators and also with economics due to their high power consumption. (See also information page on Accelerator-Driven Nuclear Energy)
A key finding from thorium fuel studies to date is that it is not economically viable to use low-enriched uranium (LEU - with a U-235 content of up to 20%) as a fissile driver with thorium fuels, unless the fuel burn-up can be taken to very high levels – well beyond those currently attainable in LWRs with zirconium cladding.
With regard to proliferation significance, thorium-based power reactor fuels would be very poor source for fissile material usable in the illicit manufacture of an explosive device. U-233 contained in spent thorium fuel contains U-232 which decays to produce very radioactive daughter nuclides and these create a strong gamma radiation field. This confers proliferation resistance by creating significant handling problems and by greatly boosting the detectability (traceability) and ability to safeguard this material.