Thorium-based nuclear power uses the nuclear fission of uranium-233, which is created from thorium, a naturally occurring element. A thorium fuel cycle has several benefits compared to a uranium fuel cycle. These include the greater availability of thorium on Earth, better physical and nuclear fuel characteristics, and less nuclear waste. Thorium fuel is harder to use for making weapons because uranium-233 is difficult to weaponize. Plutonium-239 is produced in smaller amounts and can be used in thorium reactors.

The ability to use thorium was shown on a large scale through the construction and operation of the thorium-based Light Water Breeder Reactor (LWBR) at the Shippingport Atomic Power Station. This reactor was designed to use different types of fuel. The thorium core produced 60 megawatts of electricity and operated from 1977 to 1982, generating over 2.1 billion kilowatt-hours of electricity. It also converted enough thorium-232 into uranium-233 to achieve a breeding ratio of 1.014, meaning it produced slightly more fuel than it used.

After studying thorium’s potential, scientists Ralph W. Moir and Edward Teller recommended restarting thorium research after a 30-year pause and suggested building a small prototype reactor. From 1999 to 2022, the number of non-molten-salt thorium reactors worldwide increased from zero to a few research reactors and plans for full-scale thorium reactors for national power use.

Supporters believe thorium could help create a new type of safer, cleaner nuclear power. In 2011, scientists at the Georgia Institute of Technology described thorium-based power as "a solution that could last over 1,000 years or help move toward sustainable energy sources that reduce human harm to the environment."

History

In 1940, Glenn Seaborg discovered that using thorium to create uranium-233 (U-233) could be done by bombarding thorium with neutrons in a cyclotron. During the Manhattan Project, after building the X-10 Graphite Reactor, Seaborg recognized the potential of U-233 as a material that can split to release energy. Research on U-233 continued during the project, but it was not prioritized for weapons development, as plutonium, discovered by Seaborg in February 1941, was chosen instead.

After the Atomic Energy Commission (AEC) was formed, uranium-based nuclear reactors were built to generate electricity. The first was the experimental uranium breeder EBR-I. In the United States, many reactors were light-water reactors, like the Shippingport Atomic Power Station. These designs were similar to those used in nuclear submarines. Other reactor types included liquid metal cooled reactors, such as EBR-I, and gas-cooled reactors, like Peach Bottom Unit 1 and Fort St. Vrain.

During this time, the AEC studied thorium for use in nuclear weapons and power generation. Tons of U-233 were produced from thorium in AEC reactors, with some processed at the Rocky Flats Plant. U-233 was used in the MET shot of the Operation Teapot nuclear tests.

Several commercial reactors, including Indian Point, Peach Bottom, and Fort St. Vrain, used thorium-uranium mixed oxides as fuel. Around the same time, the U.S. government built the Molten-Salt Reactor Experiment (MSRE), a prototype molten salt reactor using U-233 fuel. The MSRE, located at Oak Ridge National Laboratory, operated for about 15,000 hours from 1965 to 1969 (at a power level slightly below 8 MWth). In 1968, Glenn Seaborg, then AEC chairman, announced that a U-based reactor had been successfully developed and tested. For its final year, the reactor briefly used plutonium fluoride fuel. A proposed test with plutonium fuel was not carried out due to the project’s cancellation.

Despite its success, the MSRE was shut down in December 1969 due to pressure from Milton Shaw, director of the AEC’s Reactor Development and Testing Division. At the time, the U.S. government focused on breeder reactors to meet future nuclear fuel needs. Shaw, who preferred the liquid metal fast breeder reactor (LMFBR), pushed to end the MSRE project.

By 1973, the U.S. government largely stopped thorium research, favoring uranium technology. Uranium-fueled reactors were seen as more efficient, and thorium’s breeding ratio was considered too low to support a commercial nuclear industry. In 1970, the government started the Clinch River Breeder Reactor Project for the LMFBR, but it faced political opposition and was canceled in 1983.

In 2009, science writer Richard Martin noted that Alvin Weinberg, director of Oak Ridge and leader of the MSRE project, lost his job because he supported thorium reactors. Weinberg believed that prioritizing safety over military needs led to his retirement. However, in his 2012 book SuperFuel, Martin clarified that the AEC did not reject thorium due to weapons production goals.

Even after the MSRE’s cancellation, thorium research continued. Admiral Hyman Rickover, who developed naval nuclear propulsion, supported a thorium-fueled breeder project since 1963. In 1976, he organized the construction of a light-water breeder reactor (LWBR) at the Shippingport Atomic Power Station. This reactor, which operated until 1982, achieved a breeding ratio of 1.4%.

In Germany, the AVR pebble-bed reactor used mixed-oxide thorium-uranium TRISO fuel and operated from 1967 to 1988. Based on the AVR design, the Thorium High-Temperature Reactor (THTR-300), a 300 MWe commercial reactor, began operation in 1985. However, both reactors faced design challenges, and THTR-300 was shut down in 1989 after only four years of operation.

Despite thorium’s documented history and successful demonstration at Shippingport, many nuclear experts in 2009 were unaware of its potential. According to Chemical & Engineering News, many scientists had limited knowledge of thorium. Nuclear physicist Victor J. Stenger first learned of thorium in 2012. Former NASA scientist Kirk Sorensen, a thorium advocate, stated that if the U.S. had continued thorium research after 1974, it might have achieved energy independence by 2000.

On May 18, 2022, U.S. Senator Tommy Tuberville introduced Senate bill S.4242, the "Thorium Energy Security Act," to preserve and store U-233 for developing thorium molten-salt reactors. Sorensen had supported this effort since 2006, but the bill was not adopted by Congress.

Benefits

• Abundance. Thorium is found three times more often in Earth's crust than uranium and is nearly as common as lead and gallium. The Thorium Energy Alliance says there is enough thorium in the United States alone to power the country at its current energy level for over 1,000 years. Evans-Pritchard notes that thorium is often found as a by-product of mining for rare earth metals. Most thorium is Th-232, which can be used to make more fuel. Uranium, on the other hand, is mostly U-238, with only a small amount of U-235, which is more valuable for energy production.

• Less suitable for bombs. Thorium is harder to use for making nuclear bombs than uranium, which could help reduce the spread of nuclear weapons. Thorium is not fissile, meaning it does not split easily to cause an explosion. However, uranium-233, which is used in the thorium fuel cycle, is fissile and could be used to make a nuclear weapon. Alvin Radkowsky, a nuclear power expert, says thorium reactors produce less plutonium than standard reactors, and the plutonium made is not useful for bombs. Tests with uranium-233 showed that uranium-232, which is often mixed with it, makes the material harder to handle and increases the risk of accidents. Scientists have developed new methods to separate uranium-232 from uranium-233. In the United States, scientists at Rocky Flats successfully isolated uranium-232 decay products using chemical methods.

• Less nuclear waste. Using thorium in a liquid fluoride thorium reactor creates much less high-level nuclear waste than using uranium. Moir and Teller say this could reduce the need for long-term storage. Chinese scientists claim that thorium-based waste is up to 1,000 times less hazardous than uranium-based waste. The waste from thorium reactors becomes safe after just a few hundred years, compared to tens of thousands of years for current nuclear waste. However, the amount of activation products and fission products produced is similar between thorium and uranium fuel cycles.

• Fewer reaction startup ingredients. Moir and Teller explain that once a breeding reactor starts, it needs no other fuel except thorium because it creates most of its own fuel. Breeding reactors produce as much fissile material as they use. Non-breeding reactors, however, need additional fissile material, such as uranium-235 or plutonium, to keep the reaction going.

• Harvesting weapons-grade plutonium. The thorium fuel cycle could help produce long-term nuclear energy with less radioactive waste. Switching to thorium could also help use weapons-grade plutonium or civilian plutonium by burning it as fuel.

• No enrichment necessary. Natural thorium can be used directly as fuel, so no expensive enrichment process is needed. Similarly, U-238, which is used in the uranium-plutonium cycle, also does not require enrichment.

• Efficiency. Nobel laureate Carlo Rubbia of CERN estimates that one ton of thorium can produce as much energy as 200 tons of uranium or 3,500,000 tons of coal.

• Failsafe measures. Liquid fluoride thorium reactors are designed to prevent meltdowns. If a power failure occurs or temperatures rise too high, a fusible plug at the bottom of the reactor melts, draining the fuel into a safe underground tank.

• Mining. Mining thorium is safer and more efficient than mining uranium. Thorium is found in monazite, a type of ore that usually contains higher concentrations of thorium than uranium in its ore. This makes thorium a more cost-effective and less harmful fuel source. Thorium mining is also easier because it is often done in open pits, which do not require ventilation, unlike underground uranium mines where radon gas can be dangerous.

Disadvantages

• Extensive and costly testing, analysis, and licensing would be needed, requiring support from businesses and the government. A 2012 report by the Bulletin of the Atomic Scientists stated that using thorium fuel in current water-cooled reactors would "require too much investment and provide no clear benefits," and that "utilities would only consider thorium if it was economically beneficial."
• Producing and reprocessing thorium fuel is more expensive than using traditional solid fuel rods.
• When thorium is exposed to radiation in reactors, it becomes uranium-232, which releases gamma rays. Removing protactinium-233 during this process changes the outcome, creating uranium-233 instead of uranium-232. This makes thorium a material that could be used for both energy production and nuclear weapons.
• Thorium dioxide has a higher melting point (3350 °C) than uranium dioxide (2800 °C). This requires higher sintering temperatures or the use of non-reactive materials to create thorium dioxide-based fuel.
• Thorium is a fertile material, not a fissile one. This means it cannot start or sustain a nuclear chain reaction on its own and must be used with a separate fissile material, such as uranium or plutonium, to generate power.
• Thorium is rarely used outside of nuclear power, leading to very little demand for exploring thorium reserves for other purposes.

Proponents

Carlo Rubbia, a winner of the Nobel Prize in physics and a former leader at CERN, has always supported the use of thorium. He said, "In order to continue using nuclear power successfully, it must be greatly changed." Hans Blix, a former head of the International Atomic Energy Agency, stated, "Thorium fuel creates waste that is smaller in amount, less harmful, and does not last as long as the waste from uranium fuel."

Power projects

Research and development of thorium-based nuclear reactors, including the liquid fluoride thorium reactor (LFTR) and molten salt reactor (MSR) designs, is being conducted in the United States, United Kingdom, Germany, Brazil, India, Indonesia, China, France, the Czech Republic, Japan, Russia, Canada, Israel, Denmark, and the Netherlands. Experts from up to 32 countries attend conferences on this topic, including one held by the European Organization for Nuclear Research (CERN) in 2013. This conference focused on thorium as an alternative nuclear technology that does not require the production of nuclear waste. Hans Blix, a former leader of the International Atomic Energy Agency, supports new nuclear power technologies and states that thorium can provide a sustainable fuel supply that uses energy more efficiently.

International agencies and industry groups describe thorium-based fuel cycles as a possible long-term option for producing low-carbon electricity. These systems may have advantages such as abundant fuel supplies, high energy efficiency, and certain waste characteristics. However, most designs are still in the research, testing, or early demonstration stages.

CANDU reactors can use thorium as fuel. In 2018, the New Brunswick Energy Solutions Corporation announced that Moltex Energy would join a group working on small modular reactor technology.

At a 2011 conference, the Chinese Academy of Sciences announced that China had started a research project on thorium MSR technology. The World Nuclear Association reported that China’s program aims to develop this technology independently and gain full ownership of the intellectual property. China also plans to build two prototype reactors using components from the West and Russia. One would be a molten salt-cooled pebble-bed reactor, and the other a research molten salt reactor. The project, budgeted at $400 million, requires 400 workers. China signed an agreement with a Canadian company to improve CANDU reactors using thorium and uranium as fuel.

In 2012, China partnered with Oak Ridge National Laboratory in Tennessee to help develop its thorium reactor program. By 2014, China reduced its goal of creating a working reactor from 25 years to 10 years due to concerns about air pollution. Scientists noted that China’s interest in nuclear power has grown because of environmental issues.

By 2019, two thorium reactors were under construction in the Gobi Desert, with completion expected by 2025. China plans to use thorium reactors commercially by 2030. A 60 MWt reactor is scheduled for completion in 2029, with 10 MW of its energy used to generate electricity and the rest used to produce hydrogen.

A 2 MWt thorium prototype was nearing completion in 2021. By June 2021, China confirmed that the Gobi molten salt reactor would be completed on time, with testing beginning in September 2021. This reactor supports China’s goal of becoming carbon-neutral by 2060. China plans to build more thorium power plants in western China and in countries involved in its Belt and Road Initiative.

In August 2022, the Chinese Ministry of Ecology and Environment approved the commissioning plan for the LF1 reactor. In June 2023, the National Nuclear Safety Administration licensed the Shanghai Institute of Applied Physics to operate the TMSR-LF1, a 2 MWt reactor. In October 2023, the reactor reached full power, and by October 2024, fresh thorium was added to its fuel.

Copenhagen Atomics, a Danish company, is developing molten salt reactors that can be manufactured in large quantities. Their reactor design fits inside a shipping container and uses thorium fuel. It is designed to process nuclear waste and eventually transition to a thorium-based system. The company is testing components like valves, pumps, and control systems for molten salt reactors. In July 2024, Copenhagen Atomics announced plans to test its reactor at the Paul Scherrer Institute in Switzerland by 2026.

The German THTR-300 was a prototype reactor that used thorium as a fuel source. It operated for 432 days in the late 1980s but was shut down due to cost and technical issues. The reactor used helium as a coolant and had a pebble-bed core with uranium-235 and thorium-232 fuel.

India has the world’s largest thorium reserves but limited uranium supplies. The Indian government plans to meet up to 30% of its electricity needs with thorium by 2050. Currently, less than 2% of India’s electricity comes from nuclear power, with most generated from coal, hydroelectricity, and other renewable sources. India aims to increase nuclear power to 25% of its total electricity supply. KAMINI, India’s only thorium-based experimental reactor, produces 30 kW of thermal energy and uses uranium-233 fuel.

The 500 MWe Prototype Fast Breeder Reactor (PFBR) was initially planned for completion in 2010 but faced delays.

Thorium sources

Thorium is commonly found in a rare earth phosphate mineral called monazite. This mineral contains up to about 12% thorium phosphate, but on average, it contains 6–7%. Global monazite resources are estimated to be about 12 million tons, with two-thirds located in heavy mineral sands deposits along the south and east coasts of India. Significant deposits are also found in other countries (see table "World thorium reserves"). Monazite is a valuable source of rare earth elements (REEs), but it is not currently economical to process because the radioactive thorium produced as a byproduct would need to be stored permanently. However, if thorium-based power plants were widely used, nearly all the world's thorium needs could be met by refining monazite for its more valuable REEs.

Another estimate of reasonably assured reserves (RAR) and estimated additional reserves (EAR) of thorium comes from the OECD/NEA report "Trends in Nuclear Fuel Cycle," published in Paris, France (2001) (see table "IAEA Estimates in tons").

The numbers mentioned refer to reserves, which indicate the amount of thorium found in high-concentration deposits that can be extracted at current market prices. However, millions of times more thorium exists in Earth's 3 × 10¹⁸ tonne crust, totaling approximately 120 trillion tons. Additional, though smaller, amounts of thorium are found in deposits with intermediate concentrations. Proved reserves are a reliable measure of the total future supply of a mineral resource.

Fuel fabrication

In water-cooled reactors, the fuel used is not thorium, but instead mixed oxide fuels (MOX fuel) or thorium plutonium oxide fuels (TOX fuel). These fuels are divided into three groups:

• (Th-LEU) MOX fuels have a high amount of uranium dioxide, about 10 to 30% of the fuel's weight.
• (Th-Pu) TOX fuels contain a small amount of plutonium dioxide, about 2 to 8% of the fuel's weight.
• (Th-233U) MOX fuels have a small amount of uranium dioxide, about 2 to 5% of the fuel's weight.

First, the individual oxides that make up the fuel are turned into fine powders. These powders are then treated to reduce radioactivity and improve their ability to be shaped into solid form. The different powders are mixed and ground together to create a uniform powder. This powder is then pressed into small pellets, which are used as fuel.

Reactor types

According to the World Nuclear Association, seven types of reactors can use thorium fuel. Six of these types have been used in some way:

• Heavy water reactors (PHWRs) and the Advanced Heavy Water Reactor (AHWR)
• Aqueous homogeneous reactors (AHRs), which are designed to use naturally occurring uranium and thorium mixed in a heavy water solution. AHRs have been built and are currently operating as research reactors, according to the IAEA reactor database.
• Boiling (light) water reactors (BWRs)
• Pressurized (light) water reactors (PWRs)
• Molten salt reactors (MSRs), including liquid fluoride thorium reactors (LFTRs). Molten salt breeder reactors (MSBRs) use thorium to create more material that can split to produce energy.
• High-temperature gas-cooled reactors (HTGRs)
• Fast neutron reactors (FNRs), which are reactors where fast-moving neutrons help atoms split in a chain reaction.
• Accelerator driven reactors (ADS), which are a type of nuclear reactor design.