Radioactivity in rare earths stems from thorium and uranium, not the elements themselves. Learn who can remove them, at what cost, and what it means for
Element Profile
Thorium
Symbol
Th
Atomic number
90
Group
Actinide (associated)
Properties
Density
11.7 g/cm³
Melting point
1750 °C
Boiling point
4785 °C
Crustal abundance
10.5 ppm (upper continental crust)
Supply Risk
Supply risk
Medium
Geological scarcity
Low
Primary economic driver
Rare earth residue management & prospective nuclear fuel
Overview
About Thorium
Thorium (Th, atomic number 90) is a weakly radioactive actinide metal, and it is not one of the 17 rare earth elements. It matters to the rare earth industry because it occurs in the same ores, above all monazite. Processing leaves it in residues that must be stored or treated as radioactive material. That residue is a cost and a licensing risk for anyone building a rare earth supply chain.
An actinide that travels with rare earths
Thorium is a dense, silvery metal that tarnishes olive gray and then black when exposed to air, according to Los Alamos National Laboratory. Almost all natural thorium is thorium-232, with a half-life of about 14 billion years. Its oxide, thoria, melts at about 3,300°C, among the highest melting points of any oxide (World Nuclear Association).
The physical properties listed on this page are from the Royal Society of Chemistry periodic table. The crustal abundance shown, about 10.5 parts per million, is a U.S. Geological Survey figure for the upper continental crust. Thorium is roughly three times as abundant as uranium (RSC; WNA).
Discovery and name
Swedish chemist Jöns Jakob Berzelius identified thorium in 1829 in a mineral sample from Norway. He named it after Thor, the Norse god of thunder (LANL). Gerhard Schmidt first showed in 1898 that thorium is radioactive, and Marie Curie confirmed it (RSC).
Why rare earth producers have to manage thorium
Monazite, a rare earth phosphate mineral found in beach and river sands, is the world's main source of thorium. It holds up to about 12% thorium phosphate and averages 6% to 7% (WNA). Processing pulls the rare earths out and leaves thorium and uranium concentrated in filter cakes, sludges and tailings. Once concentrations cross legal thresholds, operators need licenses, engineered storage, monitoring and money set aside for closure, REEx reported in December 2025. In the United States, the Nuclear Regulatory Commission exempts certain rare earth products with no more than 0.25% thorium and uranium by weight, but that exemption does not cover ore or waste.
Lynas Rare Earths' plant in Malaysia produces a thorium-bearing residue from water leach purification. When Malaysia renewed Lynas's operating license for 10 years in March 2026, it required the company to stop producing radioactive waste after five years. Residue made before then must be neutralized through thorium extraction or another approved treatment, Reuters reported. In March 2026, Malaysia's science ministry said the residue must be treated to below 1 becquerel per gram (Free Malaysia Today). Below that level, Malaysian law no longer treats it as radioactive waste. In the United States, Energy Fuels said its White Mesa mill in Utah was the only licensed facility processing monazite, REEx reported in October 2026.
Thorium as a nuclear fuel: promise and limits
Thorium cannot sustain a chain reaction by itself. It is a fertile material: in a reactor, thorium-232 absorbs a neutron and eventually becomes fissile uranium-233. A thorium fuel therefore needs a fissile "driver" such as uranium-235 or plutonium to start (WNA).
The International Atomic Energy Agency's review of the thorium fuel cycle lists real advantages. A thorium-uranium-233 cycle forms far less plutonium and fewer long-lived minor actinides than uranium fuel, and thorium dioxide is chemically stable. The same review lists the drawbacks. Spent thorium fuel is hard to dissolve for reprocessing, and it carries uranium-232, whose decay products give off strong gamma radiation. Handling it requires heavily shielded, remote facilities, which raises costs. The IAEA also notes that experience with thorium fuel is limited.
The World Nuclear Association adds that cheap uranium leaves little incentive to switch fuels, and that uranium-233 counts as direct-use material under weapons safeguards. Claims that thorium is simply safer or produces less waste skip these trade-offs.
Reactors that have used thorium
Between the 1960s and 1980s, Germany ran the AVR and THTR-300 pebble-bed reactors, and the United States ran Peach Bottom and Fort St. Vrain, all on thorium-bearing fuel (IAEA). At Shippingport, Pennsylvania, a light water breeder core fueled with thorium oxide and uranium-233 operated from 1977 to 1982, according to a fuel summary report prepared for the U.S. Department of Energy.
India built its long-term nuclear plan around thorium because it has large thorium resources and limited uranium. Thorium enters at scale only in the plan's third stage, after fast breeder reactors are running (WNA). China's 2 MWt experimental molten salt reactor at Wuwei, Gansu, first went critical on October 11, 2023, and had thorium added in October 2024. In November 2025 the Shanghai Institute of Applied Physics said the reactor had converted thorium into uranium fuel, World Nuclear News reported. Companies are also developing thorium molten salt reactor designs, including Copenhagen Atomics in Denmark and the Dutch-French firm Thorizon, according to the World Nuclear Association.
Industrial uses, past and present
Concern over radioactivity has limited thorium's use in most products, and the USGS Mineral Commodity Summaries 2026 lists non-radioactive substitutes for many of them:
- Gas lamp mantles. Yttrium compounds have replaced thorium in incandescent mantles.
- Aerospace magnesium alloys. Magnesium-thorium alloys were used for high-temperature strength. A magnesium alloy with lanthanides, yttrium and zirconium can substitute. Magnesium alloys are not among the commercial uses in the USGS 2026 summary.
- Welding electrodes. Thoriated tungsten electrodes remain in some use. Oxides of cerium, lanthanum, yttrium and zirconium can replace them.
- Optics. Thorium oxide was once added to glass for high-quality camera lenses (RSC). Yttrium fluoride and other materials are used in place of thorium fluoride optical coatings.
USGS also lists catalysts, high-temperature ceramics, metal-halide lamps and nuclear medicine among current uses. It reports no thorium recycling.
Where thorium is found
The largest identified thorium resources are in India (about 850,000 tons), Brazil (630,000 tons), and Australia and the United States (600,000 tons each), USGS reported in 2026, citing World Nuclear Association estimates. The WNA also lists large resources in Egypt and Turkey.
Almost no thorium is mined for its own sake. USGS says monazite is produced mainly for its rare earths, and only a small fraction of the byproduct thorium is recovered and used. China was the leading importer of monazite in the USGS 2026 summary. No U.S. facility separated or recovered thorium in 2025, and U.S. users relied on imported thorium compounds, mostly from France and India.
What drives thorium demand and supply
Thorium supply follows rare earth demand, because more monazite processing means more thorium in residues. Demand for thorium itself depends on whether thorium-fueled reactors become commercial. Until then, the rare earth industry's concern is the cost of storing or treating the thorium its plants already produce.
Frequently asked questions
Is thorium a rare earth element?
No. Thorium is an actinide, not one of the 17 rare earth elements. It is covered alongside them because it occurs in the same ores, especially monazite, and must be managed during rare earth processing.
Why is thorium a problem for rare earth processing?
Processing monazite and similar ores concentrates thorium and uranium into residues. Above legal thresholds, those residues need licensing, engineered storage, monitoring and long-term funding, which adds cost and permitting time.
Can thorium be used as nuclear fuel?
Yes, but not on its own. Thorium-232 is fertile, so it must be paired with a fissile driver to produce uranium-233 in a reactor. The IAEA lists lower plutonium and minor actinide production as advantages, and hard reprocessing and strong gamma radiation from uranium-232 as drawbacks.
Where does thorium come from?
Mostly from monazite, a rare earth phosphate mineral found in beach and river sands. India, Brazil, Australia and the United States hold the largest identified resources, and nearly all thorium is a byproduct of rare earth recovery.
What is thorium used for today?
Industrial uses are small, because concern over its radioactivity limits them. USGS lists catalysts, high-temperature ceramics, welding electrodes, metal-halide lamps, nuclear medicine and optical coatings, and non-radioactive substitutes have been developed for many of them.
Primary Uses
- Nuclear fuel research (thorium-uranium-233 fuel cycle)
- Thoriated tungsten welding electrodes
- Catalysts and high-temperature ceramics
- Metal-halide lamps and tungsten filaments
- Nuclear medicine
- Optical coatings
Producing Projects
Companies
Found in Products
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