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Thorium (pronounced /ˈθɔəriəm/ THOHR-ee-əm) is a chemical element with the symbol Th and atomic number 90.

Thorium is a naturally occurring, slightly radioactive metal. It is estimated to be about three to four times more abundant than uranium in the Earth's crust. It has been considered a waste product in mining rare earths, so its abundance is high and cost low.

Thorium was successfully used as a breeding (fertile) source for nuclear fuel – uranium (233) in the molten-salt reactor experiment (MSR) from 1964 to 1969 (producing thermal energy for heat exchange to air or liquids), as well as in several light-water reactors using solid fuel composed of a mixture of 232Th and 233U, including the Shippingport Atomic Power Station (operation commenced 1957, decommissioned in 1982), but a thorium-uranium mix was only used at end of life to demonstrate Th-to-U breeding. Currently, the Japanese Fuji project and officials in India are advocating a thorium-based nuclear program, and a seed-and-blanket fuel utilizing thorium is undergoing irradiation testing at the Kurchatov Institute in Moscow.[2][3] Advocates of the use of thorium as the fuel source for nuclear reactors state that they can be built to operate significantly cleaner than uranium-based power plants as the waste products are much easier to handle.[4]

When used in molten-salt reactors, thorium bred to 233U removes weaponization dangers, because no uranium exists in solid form and the reactor runs continuously, with no shutdown for refuelling—all thorium and fissile uranium is consumed and any undesired gasses and uranium/plutonium isotopes are flushed out as gasses (e.g., as uranium hexafluoride) as the hot, liquid salt is pumped around the reactor/exchanger system.



Pure thorium is a silvery-white metal which is air-stable and retains its luster for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. The purest specimens often contain several tenths of a percent of the oxide. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400 °C from a face-centered cubic to a body-centered cubic structure. Powdered thorium metal is often pyrophoric and requires careful handling. When heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Thorium has the largest liquid range of any element: 2946 °C between the melting point and boiling point.[5]


Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric.[5] It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride ion.[6]


Thorium compounds are stable in the +4 oxidation state.[7]

Thorium dioxide has the highest melting point (3300 °C) of all oxides.[8]

Thorium(IV) nitrate and thorium(IV) fluoride are known in their hydrated forms: Th(NO3)4·4H2O and ThF4·4H2O, respectively. The thorium center has square planar geometry.[7] Thorium(IV) carbonate, Th(CO3)2, is also known.[7]

When treated with potassium fluoride and hydrofluoric acid, Th4+ forms the complex anion ThF2−6, which precipitates as an insoluble salt, K2ThF6.[6]

Thorium(IV) hydroxide, Th(OH)4, is highly insoluble in water, and is not amphoteric. The peroxide of thorium is rare in being an insoluble solid. This property can be utilized to separate thorium from other ions in solution.[6]

In the presence of phosphate anions, Th4+ forms precipitates of various compositions, which are insoluble in water and acid solutions.[6]

Thorium monoxide has recently been produced through laser ablation of thorium in the presence of oxygen.[9]

Main article: isotopes of thorium

Naturally occurring thorium is composed mainly of one isotope: 232Th. 230Th occurs as the daughter product of 238U decay. Twenty-seven radioisotopes have been characterized, with the most stable being 232Th with a half-life of 14.05 billion years, 230Th with a half-life of 75,380 years, 229Th with a half-life of 7340 years, and 228Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy of 7.6 eV.[10]

The known isotopes of thorium range in atomic weight from 210 u (210Th) to 236 u (236Th).[11]



Thorium is part of a certain magnesium alloys called Mag-Thor, which are used in aircraft engines, imparting high strength and creep resistance at elevated temperatures.[12][13] Thoriated magnesium was used to build the CIM-10 Bomarc missile, although concerns about radioactivity have resulted in several missiles being removed from public display.

Thorium is also used as an alloying agent in gas tungsten arc welding (GTAW) to increase the melting temperature of tungsten electrodes and improve arc stability. The electrodes labeled EWTH-1 contain 1% thorium, while the EWTH-2 contain 2%.[14] In electronic equipment, thorium coating of tungsten wire improves the electron emission of heated cathodes.[5]

Thorium is a very effective radiation shield, although it has not been used for this purpose as much as lead or depleted uranium. Thorium is a fertile material for producing nuclear fuel in a breeder reactor. Uranium-thorium age dating has been used to date hominid fossils.[5]

Thorium compounds

Thorium dioxide is a material for heat-resistant ceramics, e.g., for high-temperature laboratory crucibles. When added to glass, it helps increase refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments.[5]

Thorium dioxide (ThO2) and thorium nitrate (Th(NO3)4) were used in mantles of portable gas lights, including natural gas lamps, oil lamps and camping lights. These mantles glow with an intense white light (unrelated to radioactivity) when heated in a gas flame, and its color could be shifted to yellow by addition of cerium.[13]

Thorium dioxide was used to control the grain size of tungsten metal used for spirals of electric lamps. Thoriated tungsten elements were also found in the negative poles of magnetron tubes. Thorium was added because of its ability to emit electrons at relatively low temperatures when heated in vacuum. Those tubes were emitting electrons at microwave frequencies and were applied in microwave ovens and radars.[13]

Thorium dioxide has been used as a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking and in producing sulfuric acid. It is the active ingredient of Thorotrast, which was used as part of X-ray diagnostics. This use has been abandoned due to the carcinogenic nature of Thorotrast.[5]

Despite its radioactivity, thorium fluoride (ThF4) is used as an antireflection material in multilayered optical coatings. It has excellent optical transparency in the range 0.35–12 µm, and its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.[15] Thorium fluoride was also used in manufacturing carbon arc lamps, which provided high-intensity illumination for movie projectors and search lights.[13]

Thorium as a nuclear fuel

Main article: Thorium fuel cycle

Thorium, as well as uranium and plutonium, can be used as fuel in a nuclear reactor. A thorium fuel cycle offers several potential advantages over a uranium fuel cycle, including greater abundance on Earth, superior physical and nuclear properties of fuel, enhanced proliferation resistance, and reduced nuclear waste production.

Although not fissile itself, 232Th will absorb slow neutrons to produce 233U, which is fissile. Hence, like 238U, it is fertile. It is at least 4-5 times more abundant in Earth's crust than all isotopes of uranium combined and is fairly evenly spread around Earth[citation needed], with many countries having large supplies of it. Also, preparation of thorium fuel does not require isotopic separation.

The thorium fuel cycle creates uranium-233, which, if separated from the reactor's fuel, can be used for making nuclear weapons. This is why a liquid-fuel cycle (e.g., MSR) is preferred—only extremely hot, liquid salts ever exist in the reactor and its heat-transfer systems, preventing any access to weapons material.

Since there are no neutrons from spontaneous fission of U-233, solid U-233 can be used easily in a simple gun-type nuclear bomb design.[16] In 1977, a light-water reactor at the Shippingport Atomic Power Station was used to establish a Th232-U233 fuel cycle. The reactor worked until its decommissioning in 1982.[17][18][19] Thorium can be and has been used to power nuclear energy plants using both the modified traditional Generation III reactor design and prototype Generation IV reactor designs.

A seed-and-blanket fuel using a core of plutonium surrounded by a blanket of thorium/uranium has been undergoing testing at Moscow's Kurchatov Institute, under a 1994 agreement between the institute and McLean, Virginia-based Thorium Power Ltd. Russian government-owned nuclear design firm Red Star formed an agreement with Thorium Power in 2007 to continue work on scaling up the test fuel rods to commercial use and licensing in VVER-1000 reactors. This assembly could achieve a more efficient disposal method of weapons-grade plutonium than the mixed-oxide disposal method, especially[citation needed] with the 2009 decision by the US to shelve the Yucca Mountain nuclear waste storage program highlighting the issue of what to do with all the plutonium left over from decommissioned nuclear weapons.[20] Thorium Power, with offices in London, Dubai, and Moscow and with Dr. Hans Blix serving as an advisor, also advises the United Arab Emirates on their fledgling nuclear program. They are awaiting the finalization of the US-India nuclear 1-2-3 Agreement to complete a joint-venture with Punj Lloyd, an Indian engineering firm with nuclear reactor construction ambitions.[21]

Unlike its use in MSRs, when using solid thorium in modified light water reactor (LWR) problems include: the undeveloped technology for fuel fabrication; in traditional, once-through LWR designs potential problems in recycling thorium due to highly radioactive 228Th; some weapons proliferation risk due to production of 233U; and the technical problems (not yet satisfactorily solved) in reprocessing. Much development work is still required before the thorium fuel cycle can be commercialized for use in LWR. The effort required has not seemed worth it while abundant uranium is available, but geopolitical forces (e.g. India looking for indigenous fuel) as well as uranium production issues, proliferation concerns, and concerns about the disposal/storage of radioactive waste are starting to work in its favor. In 2008, Senator Harry Reid (D-Nevada) and Senator Orrin Hatch (R-Utah) introduced the Thorium Energy Independence and Security Act of 2008, which would mandate a US Department of Energy initiative to examine the commercial use of thorium in US reactors.[22] The bill, however, did not reach a full Senate vote.

The thorium fuel cycle, with its potential for breeding fuel without fast neutron reactors, holds considerable potential long-term benefits. Thorium is significantly more abundant than uranium, and is a key factor in sustainable nuclear energy. Perhaps more importantly, thorium produces one to two orders of magnitude less long-lived transuranics than uranium fuel cycles, though the long-lived actinide protactinium-231 is produced, and the amount of fission products is similar..

An early effort to use a thorium fuel cycle took place at Oak Ridge National Laboratory in the 1960s. An experimental reactor was built based on MSR technology to study the feasibility of such an approach, using thorium-fluoride salt kept hot enough to be liquid, thus eliminating the need for fabricating fuel elements. This effort culminated in the Molten-Salt Reactor Experiment that used 232Th as the fertile material and 233U as the fissile fuel. This reactor was operated successfully for about five years. However, due to a lack of funding, the MSR program was discontinued in 1976. Nowadays this design is considered as Generation IV reactor.

India's Kakrapar-1 reactor is the world's first reactor which uses thorium rather than depleted uranium to achieve power flattening across the reactor core.[23] India, which has about 25% of the world's thorium reserves, is developing a 300 MW prototype of a thorium-based Advanced Heavy Water Reactor (AHWR). The prototype is expected to be fully operational by 2011, following which five more reactors will be constructed.[24] Considered to be a global leader in thorium-based fuel, India's new thorium reactor is a fast-breeder reactor and uses a plutonium core rather than an accelerator to produce neutrons. As accelerator-based systems can operate at sub-criticality they could be developed too, but that would require more research.[25] India currently envisages meeting 30% of its electricity demand through thorium-based reactors by 2050.[26]

In 2007, Norway was debating whether or not to focus on thorium plants because of the large deposits of thorium ores in the country, particularly at Fensfeltet near Ulefoss in Telemark county.

The primary fuel of the HT3R Project near Odessa, Texas, USA will be ceramic-coated thorium beads.[27]

However, the best results occur with molten-salt reactors MSRs, such as ORNL's LFTR, which have built-in negative-feedback reaction rates, due to salt expansion and thus reactor throttling via load. This is a great safety advantage, since no emergency cooling system is needed, which is both expensive and adds thermal inefficiency. In fact, an MSR was chosen as the base design for the 1960s DoD Atomic Plane largely because of its great safety advantages, even under aircraft maneuvering. In the basic design, an MSR generates heat at higher temperatures, continuously, and without refuelling shutdowns, so it can provide hot air to a more efficient (Brayton Cycle) turbine. An MSR run this way is about 30% better in thermal efficiency than common thermal plants, whether combustive or traditional solid-fuelled nuclear.[28],


M. T. Esmark found a black mineral on Løvøy Island, Norway and gave a sample to Professor Jens Esmark, a noted mineralogist who was not able to identify it, so he sent a sample to the Swedish chemist Jöns Jakob Berzelius for examination in 1828.[29][30][31] Berzelius analyzed it and named it after Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the gas mantle in 1885.

Thorium was first observed to be radioactive in 1898, independently, by Polish-French physicist Marie Curie and English chemist Gerhard Carl Schmidt.[32][33][34] Between 1900 and 1903, Ernest Rutherford and Frederick Soddy showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half life as one of the outcomes of the alpha particle experiments that led to their disintegration theory of radioactivity.[35]

The crystal bar process (or Iodide process) was discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925 to produce high-purity metallic thorium.[36]

The name ionium was given early in the study of radioactive elements to the 230Th isotope produced in the decay chain of 238U before it was realized that ionium and thorium were chemically identical. The symbol Io was used for this supposed element.

Monazite, a rare-earth-and-thorium phosphate mineral, is the primary source of the world's thorium
See also: Category:thorium minerals

Thorium is found in small amounts in most rocks and soils, where it is about four times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 12 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO4), thorianite (ThO2 + UO2) and monazite. Thorianite is the most common and may contain up to about 12% thorium oxide. Thorium-containing monazite(Ce) occurs in all continents.[5][37]

232Th decays very slowly (its half-life is comparable to the age of the Universe) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than 232Th, though on a mass basis they are negligible.

Thorium extraction
Main article: Monazite

Thorium has been extracted chiefly from monazite through a complex multi-stage process. The monazite sand is dissolved in hot concentrated sulfuric acid (H2SO4). Thorium is extracted as an insoluble residue into an organic phase containing an amine. Next it is separated or "stripped" using an ion such as nitrate, chloride, hydroxide, or carbonate, returning the thorium to an aqueous phase. Finally, the thorium is precipitated and collected.[38]

Several methods are available for producing thorium metal: it can be obtained by reducing thorium oxide with calcium, by electrolysis of anhydrous thorium chloride in a fused mixture of sodium and potassium chlorides, by calcium reduction of thorium tetrachloride mixed with anhydrous zinc chloride, and by reduction of thorium tetrachloride with an alkali metal.[5]


Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.[39] There are two sets of estimates that define world thorium reserves, one set by the US Geological Survey (USGS) and the other supported by reports from the OECD and the International Atomic Energy Agency (the IAEA). Under the USGS estimate, Australia and India have particularly large reserves of thorium. India and Australia are believed to possess about 300,000 tonnes each; i.e. each country possessing 25% of the world's thorium reserves.[40] However, in the OECD reports, estimates of Australian's Reasonably Assured Reserves (RAR) of thorium indicate only 19,000 tonnes and not 300,000 tonnes as indicated by USGS. The two sources vary wildly for countries such as Brazil, Turkey, and Australia. However, both reports appear to show some consistency with respect to India's thorium reserve figures, with 290,000 tonnes (USGS) and 319,000 tonnes (OECD/IAEA). Furthermore the IAEA report mentions that India possesses two thirds (67%) of global reserves of monazite, the primary thorium ore. The IAEA also states that recent reports have upgraded India's thorium deposits up from approximately 300,000 tonnes to 650,000 tonnes.[41] Therefore, the IAEA and OECD appear to conclude that Brazil and India may actually possess the lion's share of world's thorium deposits.

* The prevailing estimate of the economically available thorium reserves comes from the US Geological Survey, Mineral Commodity Summaries (1997–2006):[42][43]

Country Th reserves (tonnes) Th reserve base (tonnes)
Australia 300,000 340,000
India 290,000 300,000
Norway 170,000 180,000
United States 160,000 300,000
Canada 100,000 100,000
South Africa 35,000 39,000
Brazil 16,000 18,000
Malaysia 4,500 4,500
Other Countries 95,000 100,000
World Total 1,200,000 1,400,000

Note: The OECD/NEA report notes that the estimates (that the Australian figures are based on) are subjective, due to the variability in the quality of the data, a lot of which is old and incomplete.[44] Adding to the confusion are subjective claims made by the Australian government (in 2009, through their "Geoscience" department) that combine the Reasonably Assured Reserves (RAR) estimates with "inferred" data (i.e. subjective guesses). This strange combined figure of RAR and "guessed" reserves yields a figure, published by the Australian government, of 489,000 tonnes.[44] However using the same criteria for Brazil or India would yield reserve figures of between 600,000 to 1,300,000 tonnes for Brazil and between 300,000 to 600,000 tonnes for India. Irrespective, of isolated claims by the Australian government, the most credible third-party and multi-lateral reports, those of the OECD/IAEA and the USGS, consistently report high thorium reserves for India while not doing the same for Australia.

* Another estimate of Reasonably Assured Reserves (RAR) and Estimated Additional Reserves (EAR) of thorium comes from OECD/NEA, Nuclear Energy, "Trends in Nuclear Fuel Cycle", Paris, France (2001):[45]

Country RAR Th (tonnes) EAR Th (tonnes)
Brazil 606,000 700,000
Turkey 380,000 500,000
India 319,000 —
United States 137,000 295,000
Norway 132,000 132,000
Greenland 54,000 32,000
Canada 45,000 128,000
Australia 19,000 —
South Africa 18,000 —
Egypt 15,000 309,000
Other Countries 505,000 —
World Total 2,230,000 2,130,000

Dangers and biological roles

Powdered thorium metal is pyrophoric and will often ignite spontaneously in air. Natural thorium decays very slowly compared to many other radioactive materials, and the alpha radiation emitted cannot penetrate human skin meaning owning and handling small amounts of thorium, such as a gas mantle, is considered safe. Exposure to an aerosol of thorium can lead to increased risk of cancers of the lung, pancreas and blood, as lungs and other internal organs can be penetrated by alpha radiation. Exposure to thorium internally leads to increased risk of liver diseases.

The element has no known biological role.
See also: actinides in the environment and Thorotrast

Chemical thermodynamics of thorium, M. H. Rand

See also

* Periodic table
* Nuclear reactor
* Decay chain
* Sylvania Electric Products explosion
* Energy amplifier
* Molten salt reactor


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External links

* WebElements.com — Thorium
* European Nuclear Society — Natural Decay Chains
* ATSDR CDC ToxFAQs: health questions about thorium
* FactSheet on Thorium, World Nuclear Association

Chemistry Encyclopedia

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