Thorium

iso	NA	half-life	DM	DE (MeV)	DP
²²⁸Th	trace	1.9116 years	α	5.520	²²⁴Ra
²²⁹Th	syn	7340 years	α	5.168	²²⁵Ra
²³⁰Th	trace	75380 years	α	4.770	²²⁶Ra
²³¹Th	trace	25.5 hours	β	0.39	²³¹Pa
²³²Th	100%	1.405×10¹⁰ years	α	4.083	²²⁸Ra
²³⁴Th	trace	24.1 days	β	0.27	²³⁴Pa

Thorium ( /ˈθɔəriəm/ thohr-ee-əm) is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder.

In nature, virtually all thorium is found thorium-232, and it decays by emitting an alpha particle, and has a half-life of about 14.05 billion years (other, trace-level isotopes of thorium are short-lived intermediates of decay chains). It is estimated to be about three times more abundant than uranium in the Earth's crust and is a by-product of the extraction of rare earths from monazite sands. Thorium was formerly used commonly as (for example) the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity.

The molten-salt reactor experiment conducted in the United States from 1964 to 1969 used thorium-232 to breed uranium-233; most of the test reactors were closed down.

Countries like Russia, India and China now have plans to use thorium for their nuclear power for various reasons, including its safety benefits, its high absolute abundance and relative abundance compared to uranium.

Characteristics

Physical properties

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.^[2] 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 1360 °C from a face-centered cubic to a body-centered cubic structure; a body-centered tetragonal lattice form exists at high pressure with impurities driving the exact transition temperatures and pressures.^[2] 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 one of the largest liquid temperature ranges of any element, with 2946 °C between the melting point and boiling point.^[3] Thorium metal is paramagnetic with a ground state of 6d²7s².^[2]

Chemical properties

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric acid.^[3] It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride ion.^[4]

Compounds

Thorium compounds are stable in the +4 oxidation state.^[5]

Thorium dioxide has the highest melting point (3300 °C) of all oxides.^[6]

Thorium(IV) nitrate and thorium(IV) fluoride are known in their hydrated forms: Th(NO₃)₄·4H₂O and ThF₄·4H₂O, respectively.^[5] Thorium(IV) carbonate, Th(CO₃)₂, is also known.^[5]

When treated with potassium fluoride and hydrofluoric acid, Th⁴⁺ forms the complex anion ThF2−
6, which precipitates as an insoluble salt, K₂ThF₆.^[4]

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.^[4]

In the presence of phosphate anions, Th⁴⁺ forms precipitates of various compositions, which are insoluble in water and acid solutions.^[4]

Thorium monoxide has recently been produced through laser ablation of thorium in the presence of oxygen.^[7]

Isotopes

Main article: Isotopes of thorium

Twenty-seven radioisotopes have been characterized, with a range in atomic weight from 210 u (²¹⁰Th) to 236 u (²³⁶Th).^[8] The most stable isotopes are:

²³²Th with a half-life of 14.05 billion years, it represents all but a trace of naturally occurring thorium.
²³⁰Th with a half-life of 75,380 years. Occurs as the daughter product of ²³⁸U decay.
²²⁹Th with a half-life of 7340 years. It has a nuclear isomer (or metastable state) with a remarkably low excitation energy of 7.6 eV.^[9]
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.

Applications

Thorium

Thorium is a component of the magnesium alloy called Mag-Thor, used in aircraft engines and imparting high strength and creep resistance at elevated temperatures.^[10]^[11] 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.^[12] The electrodes labeled EWTH-1 contain 1% thorium, while the EWTH-2 contain 2%.^[13] In electronic equipment, thorium coating of tungsten wire improves the electron emission of heated cathodes.^[3]

Thorium is a very effective radiation shield, although it has not been used for this purpose as much as lead or depleted uranium. Uranium-thorium age dating has been used to date hominid fossils,^[3] seabeds, and mountain ranges.^[14] Environmental concerns related to radioactivity led to a sharp decrease in demand for nonnuclear uses of thorium in the 2000s.^[14]

Thorium compounds

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.^[11]

Thorium dioxide is a material for heat-resistant ceramics, e.g., for high-temperature laboratory crucibles.^[12] 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.^[3] The radiation from these lenses can self-darken (yellow) them over a period of years and degrade film, but the health risks are minimal.^[15] Yellowed lenses may be restored to their original colorless state with lengthy exposure to intense UV light.

Thorium dioxide was used to control the grain size of tungsten metal used for spirals of electric lamps. Thoriated tungsten elements are found in the filaments of magnetron tubes. Thorium is added because of its ability to emit electrons at relatively low temperatures when heated in vacuum. Those tubes generate microwave frequencies and are applied in microwave ovens and radars.^[11]

Thorium dioxide has been used as a catalyst in the conversion of ammonia to nitric acid,^[12] 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.^[3]

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.^[16] Thorium fluoride was also used in manufacturing carbon arc lamps, which provided high-intensity illumination for movie projectors and search lights.^[11]

Thorium as a nuclear fuel

Benefits and challenges

Thorium can be used as fuel in a nuclear reactor, and it is a fertile material, which allows it to be used to produce nuclear fuel in a breeder reactor. In 1997, the U.S. Energy Department underwrote research into thorium fuel, and research was also begun in 1996 by the International Atomic Energy Agency (IAEA), to study the use of thorium reactors. Nuclear scientist Alvin Radkowsky of Tel Aviv University in Israel founded a consortium to develop thorium reactors, which included other companies: Raytheon Nuclear Inc., Brookhaven National Laboratory, and the Kurchatov Institute in Moscow.^[17] Radkowsky was chief scientist in the U.S. nuclear submarine program directed by Admiral Hyman Rickover and later headed the design team which built the USA's first civilian nuclear power plant at Shippingport, Pennsylvania, which was a scaled-up version of the first naval reactor.^[17] Some countries, including India, are now investing in research to build thorium-based nuclear reactors. A 2005 report by the International Atomic Energy Agency discusses potential benefits along with the challenges of thorium reactors.^[18] India has also made thorium-based nuclear reactors a priority with its focus on developing fast breeder technology.^[19]^[20]

Some benefits of thorium fuel when compared with uranium were summarized as follows:^[21]

Weapons-grade fissionable material (²³³U) is harder to retrieve safely and clandestinely from a thorium reactor;

Thorium produces 10 to 10,000 times less long-lived radioactive waste;

The fissionable thorium cycle uses 100% of the isotope as coming out of the ground, which does not require enrichment, whereas the fissile uranium cycle depends on the only 0.7% fissile U-235 of the natural uranium. The same cycle could also use the fissionable U-238 component of the natural uranium, and also contained in the depleted reactor fuel;

Thorium cannot sustain a nuclear chain reaction without priming ^[22] so fission stops by default.

However, when used in a breeder like reactor, unlike uranium-based breeder reactors, thorium requires irradiation and reprocessing before the above-noted advantages of thorium-232 can be realized, which makes thorium fuels initially more expensive than uranium fuels.^[14] But experts note that "the second thorium reactor may activate a third thorium reactor. This could continue in a chain of reactors for a millennium if we so choose." They add that because of thorium's abundance, it will not be exhausted in 1,000 years.^[23]

The Thorium Energy Alliance (TEA), an educational advocacy organization, emphasizes that "there is enough thorium in the United States alone to power the country at its current energy level for over 1,000 years."^[24]

Thorium energy fuel cycle

Main article: Thorium fuel cycle

Although not fissile itself, ²³²Th will absorb slow neutrons to produce, after two beta decays, ²³³U, which is fissile.^[14] Hence, like ²³⁸U, it is fertile. Also, preparation of thorium fuel does not require isotopic separation.

The thorium fuel cycle creates ²³³U, which, if separated from the reactor's fuel, can be used for making nuclear weapons. This is why a liquid-fuel cycle (e.g., Molten Salt Reactor or MSR) is preferred — only a limited amount of ²³³U ever exists in the reactor and its heat-transfer systems, preventing any access to weapons material; however the neutrons produced by the reactor can be absorbed by a thorium or uranium blanket and fissile ²³³U or ²³⁹Pu produced. Also, the ²³³U could be continuously extracted from the molten fuel as the reactor is running. Neutrons from the decay of uranium-233 can be fed back into the fuel cycle to start the cycle again.^[14]

The neutron flux from spontaneous fission of ²³³U is negligible. ²³³U can thus be used easily in a simple gun-type nuclear bomb design.^[25] In 1977, a light-water reactor at the Shippingport Atomic Power Station was used to establish a ²³²Th-²³³U fuel cycle. The reactor worked until its decommissioning in 1982.^[26]^[27]^[28] 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. The use of thorium as an alternative fuel is one innovation being explored by the International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO),^[29] conducted by the International Atomic Energy Agency (IAEA).

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.

Commercial nuclear power station

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.^[30] 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, after which five more reactors will be constructed.^[31] 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.^[32] India currently envisages meeting 30% of its electricity demand through thorium-based reactors by 2050.^[33]

Existing thorium energy projects

The German THTR-300 was the first commercial power station powered almost entirely with Thorium. India's 300 MWe AHWR CANDU type reactor will begin construction in 2011. The design envisages a start up with reactor grade plutonium which will breed U-233 from Th-232. After that the input will only be thorium for the rest of the reactor's design life.^[34]

The primary fuel of the HT³R Project near Odessa, Texas, USA will be ceramic-coated thorium beads. The earliest date the reactor will become operational in 2015.^[35]

Best results occur with molten salt reactors (MSRs), such as ORNL's liquid fluoride thorium reactor (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 nuclear aircraft 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.^[24]

In 2010, Congressman Joe Sestak added funding for research and development of a destroyer-sized reactor using thorium.^[36]

CANDU reactors of Atomic Energy Canada Limited are capable of using thorium as a fuel source.^[37]^[38]

At the 2011 annual conference of the Chinese Academy of Sciences it was announced that "China has initiated a research and development project in thorium molten-salt reactor technology."^[39]^[40]

Projects combining uranium and thorium

Fort St. Vrain Generating Station, a demo HTGR in Colorado, USA, operating from 1977 until 1992, employed enriched uranium fuel that also contained thorium. This resulted in high fuel efficiency because the thorium was converted to uranium and then burnt.

History

Swedish chemist Jöns Jakob Berzelius analyzed a mineral from the Falun district in 1828 and determined that it contained a new element, which he named thorium after Thor, the Norse god of thunder.^[12] Analysis 10 years later found that the mineral was xenotime (Y P). Morten Thrane Esmark found a black mineral on Løvøy Island, Norway and gave a sample to his father Jens Esmark, a noted mineralogist. The elder Esmark was not able to identify it and sent a sample to Berzelius for examination in 1828.^[41]^[42] Berzelius analyzed it and gave it the same name as the misidentified sample of xenotime. The metal had no practical uses until Carl Auer von Welsbach invented the gas mantle in 1885.^[12]

Thorium was first observed to be radioactive in 1898, independently, by Polish-French physicist Marie Curie and German chemist Gerhard Carl Schmidt.^[43]^[44]^[45] 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.^[46]

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.^[47]

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.

Occurrence

Thorium is found in small amounts in most rocks and soils; it is three times more abundant than tin in the Earth's crust and is about as common as lead.^[48] 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 a rare mineral and may contain up to about 12% thorium oxide. Monzonite contains 2.5% thorium, allanite has 0.1 to 2% thorium and zircon can have up to 0.4% thorium.^[49] Thorium-containing minerals occur on all continents.^[3]^[50]^[51] Thorium is several times more abundant in Earth's crust than all isotopes of uranium combined and thorium-232 is several hundred times more abundant than uranium-235.^[14]

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.^[52]

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.^[3]

Reserves

Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.^[53] 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, USA, Australia and India have particularly large reserves of thorium.

Both the IAEA and OECD appear to conclude that India may actually possess the lion's share of world's thorium deposits. The Government of India's estimate puts the reserve at 846,477 tonnes.^[54] The logic for this assessment is as follows.

OECD assessment

India and Australia are believed to possess about 300,000 tonnes each; i.e. each country possessing 25% of the world's thorium reserves.^[55] However, in the OECD reports, estimates of Australia'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).

IAEA assessment

The IAEA's 2005 report estimates India's reasonably assured reserves of thorium at 319,000 tonnes, but mentions recent reports of India's reserves at 650,000 tonnes.^[56]

The prevailing estimate of the economically available thorium reserves comes from the US Geological Survey, Mineral Commodity Summaries (1996–2010):^[51]^[57]

American estimates in tonnes (2010)
Country	Reserves
United States	440,000
Australia	300,000
Brazil	16,000
Canada	100,000
India	290,000 to 650,000
Malaysia	4,500
South Africa	35,000
Other Countries	90,000
World Total	1,300,000 to 1,660,000

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.^[58] Adding to the confusion are subjective claims made by the Australian government (in 2009, through its "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.^[58] 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):^[59]

IAEA Estimates in tonnes (2005)
Country	RAR Th	EAR Th
Australia	489,000	19%
USA	400,000	15%
Turkey	344,000	13%
India	319,000	12%
Venezuela	300,000	12%
Brazil	302,000	12%
Norway	132,000	5%
Egypt	100,000	4%
Russia	75,000	3%
Greenland	54,000	2%
Canada	44,000	2%
South Africa	18,000	1%
"Other countries"	33,000	1%
"World total"	2,610,000

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.

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Bibliography

Wickleder, Mathias S.; Fourest, Blandine; Dorhourt, Peter K. (2006). "Thorium". In Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean. The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Springer Science+Business Media. ISBN 1-4020-3555-1.