Thorium
What is Thorium?
Thorium (chemical symbol Th, atomic number 90) is a naturally occurring, weakly radioactive metallic element. It was discovered in 1828 by Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of thunder. Thorium is a member of the actinide series on the periodic table.
In nature, thorium exists almost entirely as Thorium-232 (Th-232), with a half-life of approximately 14.05 billion years — roughly three times the age of the Earth. This extreme stability means thorium ore poses very little immediate radiation hazard during mining or handling.
Abundance and Distribution
Thorium is approximately 3 to 4 times more abundant than uranium in the Earth's crust, with an average crustal concentration of around 6–10 parts per million. It is found in many common minerals, most notably:
- Monazite — a phosphate mineral that is the primary commercial source of thorium, also rich in rare earth elements
- Thorite (ThSiO₄)
- Thorianite (ThO₂)
- Bastnäsite
Monazite sand deposits — heavy mineral sands found on beaches and in riverbeds — are especially rich sources and are found in Australia, India, Brazil, the United States, South Africa, Canada, and throughout Southeast Asia.
World Thorium Reserves (Estimated)
| Country | Estimated Reserves (tonnes) |
|---|---|
| India | 846,000 |
| Brazil | 632,000 |
| Australia | 595,000 |
| United States | 440,000 |
| Egypt | 380,000 |
| Norway | 132,000 |
| Turkey | 344,000 |
| Venezuela | 300,000 |
| Canada | 172,000 |
| South Africa | 148,000 |
| Other countries | ~750,000 |
| World Total | ~6,355,000+ |
Note: China's thorium reserves are not included in most public IAEA estimates, but are believed to be substantial.
Physical and Chemical Properties
- Atomic weight: 232.038
- Melting point: 1,750 °C (3,182 °F)
- Boiling point: 4,788 °C (8,650 °F)
- Density: 11.7 g/cm³
- Appearance: Silvery-white metal, tarnishes to grey or black on exposure to air
- Radioactive decay: Alpha emitter; decays very slowly through a long chain eventually to lead-208
Thorium as Nuclear Fuel
Thorium-232 is fertile but not fissile. It cannot undergo sustained nuclear fission by thermal neutrons without first being converted into fissile U-233. This conversion is achieved by neutron capture and subsequent radioactive decay (see: Thorium Fuel Cycle).
One tonne of thorium, if fully converted and fissioned in a well-designed reactor, can yield energy equivalent to approximately:
- 200 tonnes of uranium oxide (for use in a conventional LWR)
- 3.5 million tonnes of coal
The global identified thorium resource of over 6 million tonnes therefore represents an essentially inexhaustible energy supply if used in breeder reactors — sufficient to power human civilisation for thousands of years at current consumption levels.
Non-Energy Uses
Before its nuclear potential was recognised, thorium was used commercially in:
- Gas mantles: Thorium oxide was used in incandescent gas mantles throughout the late 19th and early 20th centuries.
- Refractory materials: Thorium oxide (ThO₂) has an extremely high melting point (3,300 °C) and was used in high-temperature crucibles and heating elements.
- Alloys: Small quantities of thorium improve the high-temperature strength of magnesium and other alloys.
- Optics: Thoriated lenses (camera and telescope glass) were produced until the 1970s. Photographers still use vintage thoriated lenses today.
- Welding electrodes: Thoriated tungsten electrodes (2% ThO₂) produce a more stable arc and are still used in TIG welding, though alternatives are increasingly being adopted due to radioactivity concerns.
