The chemical symbol “Th” on the periodic table represents Thorium, a naturally occurring radioactive metal. Classified as an actinide, Thorium is recognized for its complex nuclear properties. It has garnered significant attention as a potential alternative nuclear fuel source. This element is widely distributed in the Earth’s crust and offers unique advantages that could reshape the global energy landscape.
Fundamental Properties of Thorium
Thorium holds the atomic number 90 and an atomic weight of approximately 232.0377. In its pure metallic form, thorium is a soft, silvery-white material, similar in density to lead. It quickly develops a dull black tarnish when exposed to air.
All known isotopes of this element are radioactive, meaning their nuclei are unstable and decay over time. The most abundant isotope, Thorium-232 (Th-232), makes up virtually all naturally occurring thorium. Th-232 is notable for its extremely long half-life, spanning about 14 billion years, a duration comparable to the age of the universe. This slow decay rate makes Th-232 a primordial nuclide, still present in large quantities since the Earth’s formation.
Natural Occurrence and Abundance
Thorium is a lithophile element, meaning it primarily bonds with oxygen and is concentrated in the Earth’s rocky crust. It is estimated to be three to four times more abundant than uranium, with an average concentration of about six parts per million in soil. The element is typically extracted from rare-earth minerals, such as the phosphate mineral monazite, often found in heavy mineral sands.
The element was discovered in 1828 by the Swedish chemist Jöns Jacob Berzelius, who named it after the Norse god of thunder, Thor. Significant global reserves are unevenly distributed, with large deposits located in countries like India, Brazil, and the United States. These reserves are often found as a by-product of mining for other rare-earth elements.
Thorium’s Role in Nuclear Energy
The primary interest in thorium stems from its potential to power nuclear reactors through the thorium fuel cycle. Thorium-232 is not fissile itself, meaning it cannot sustain a chain reaction directly, but it is a fertile material. The cycle begins when a Th-232 atom absorbs a neutron, transforming it into Thorium-233.
This unstable isotope quickly undergoes two successive beta decays to become Uranium-233 (U-233), a highly effective fissile material. The U-233 can then fission, releasing energy and more neutrons to continue the cycle.
A key advantage of the thorium cycle is that it produces significantly less long-lived radioactive waste, specifically minor actinides, compared to the uranium-plutonium cycle. Furthermore, the inherent design of some proposed thorium reactors, such as Molten Salt Reactors (MSRs), offers passive safety features. If the reactor overheats, a plug of frozen salt can melt, draining the liquid fuel into a safe storage tank and halting the nuclear reaction without human intervention.
Despite these advantages, thorium has not yet replaced uranium as the dominant fuel source. This is partly because it requires an initial input of fissile material like U-235 or plutonium to begin the breeding process. The necessary technologies for reprocessing and refabricating the resulting U-233 fuel are complex and require high initial investment. Since uranium resources were considered sufficient, the political and economic incentive to overcome these technological hurdles was limited, slowing commercial deployment.
Non-Nuclear Industrial Applications
While its nuclear potential receives the most attention, thorium has also been used in various industrial applications. Historically, thorium dioxide was used in the production of Welsbach gas mantles, which glowed brightly when heated by a flame. This application has largely been phased out due to concerns about the element’s radioactivity.
Thorium is still used in certain high-performance materials where its properties are beneficial. Its oxide is incorporated into specialized optical glass for high-quality camera and telescope lenses, increasing the glass’s refractive index and reducing dispersion. It is also an alloying agent in aerospace materials, such as magnesium-thorium alloys, used to strengthen metals. A current use is in the electrodes for TIG (Tungsten Inert Gas) welding, though non-radioactive alternatives are increasingly being adopted.