What Is Thorium?

Thorium is a naturally occurring, weakly radioactive metallic element with atomic number 90 and the symbol Th. It belongs to the actinide series and is approximately four times more abundant in the Earth's crust than uranium, occurring at an average concentration of around 10 parts per million. Its principal naturally occurring isotope, thorium-232 (Th-232), has a half-life of about 14 billion years, comparable to the age of the universe, which makes it essentially stable for engineering purposes. Thorium has attracted sustained scientific interest as a candidate nuclear fuel material because of its fertile properties, favorable chemical characteristics, and potential for lower waste production compared to uranium-based fuel cycles.

Thorium's relevance to nuclear engineering was recognized in the mid-twentieth century, with significant early research conducted at American and Indian national laboratories. India, home to some of the world's largest thorium reserves in its coastal mineral sands, has maintained thorium fuel research as a strategic priority. The International Atomic Energy Agency has catalogued thorium's potential and challenges in a series of technical reports covering the thorium fuel cycle.

Fertile Material and the Thorium Fuel Cycle

Unlike uranium-235, thorium-232 is not itself fissile and cannot sustain a chain reaction directly. It is, however, fertile: when Th-232 absorbs a neutron, it transmutes through protactinium-233 to uranium-233 (U-233), a fissile isotope that can then sustain fission. This conversion process requires an initial fissile driver, typically U-235 or plutonium, to provide the neutrons necessary to breed U-233. The resulting fuel cycle is described in detail in the IAEA's technical documentation on the thorium fuel cycle, which identifies higher thermal neutron cross sections and reduced production of long-lived transuranic waste as among the principal advantages. Because U-233 produced in thorium fuels is contaminated with U-232, whose decay products emit intense gamma radiation, the material presents significant proliferation resistance relative to plutonium.

Material Properties

Thorium dioxide (ThO2), the oxide form used in nuclear fuel fabrication, exhibits a melting point of approximately 3300 degrees Celsius, among the highest of any known oxide. Its thermal conductivity exceeds that of uranium dioxide (UO2), and its lower coefficient of thermal expansion translates into less mechanical stress under irradiation. These properties contribute to greater dimensional stability in the reactor core over long irradiation periods. Thorium metal itself is ductile and silvery in appearance when freshly processed, oxidizing on exposure to air to form a grey-black surface layer. The World Nuclear Association's thorium reference documents the comparative physical and nuclear data for thorium versus uranium fuels.

Reactor Concepts and Research

Several reactor architectures are under investigation for thorium fuel cycles. Molten salt reactors (MSRs) dissolve thorium compounds in a fluoride salt that circulates through the core, combining the fuel and coolant in a single medium. This design allows continuous fuel processing and operates at near-atmospheric pressure, eliminating pressurized-water safety concerns. Pressurized heavy-water reactors and advanced CANDU designs have also demonstrated thorium fuel viability in test campaigns. The American Scientist article on thorium as a nuclear fuel reviews the technical and policy context of these efforts, including the challenge of developing the reprocessing infrastructure needed to extract bred U-233.

Applications

Thorium has applications in a range of fields, including:

• Nuclear power generation through breeder reactor and molten salt reactor fuel cycles
• High-temperature refractory materials, including thoriated tungsten welding electrodes
• Neutron source targets and nuclear research reactor fuel pins
• Historical use in gas mantle lighting and specialized optical glass fabrication
• National energy security planning in thorium-rich nations such as India and Brazil