Thorium (pronEng|ˈθɔːriəm) is a
chemical elementwith the symbol Th and atomic number90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium.
When pure, thorium is a silvery-white metal that retains its luster for several months. However, when it is exposed to oxygen, thorium slowly tarnishes in air, becoming grey and eventually black.
Thorium dioxide(ThO2), also called thoria, has the highest melting point of any oxide (3300°C). [ cite book | last = Emsley | first = John | title = Nature's Building Blocks | edition = (Hardcover, First Edition) | publisher = Oxford University Press| date = 2001 | pages = page 441 | id = ISBN 0198503407 ] 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 K between the melting point and boiling point.
Actinides in the environmentfor details of the environmental aspects of thorium.
Applications of thorium:
* As an
alloyingelement in magnesium, used in aircraft engines, imparting high strength and creep resistance at elevated temperatures.
* Thorium is used to coat
tungstenwire used in electronic equipment, improving the electronemission of heated cathodes.
* Thorium is an alloying agent used in
GTAW("Gas Tungsten Arc Welding") to increase the melting temperature of tungsten electrodes and improve arc stability.
* Uranium-thorium age dating has been used to date hominid
* As a
fertile materialfor producing nuclear fuel. In particular, the proposed energy amplifierreactor design would employ thorium. Since thorium is more abundant than uranium, some nuclear reactordesigns incorporate thorium in their fuel cycle.
* Thorium is a very effective
radiation shield, although it has not been used for this purpose as much as leador depleted uranium.
* Thorium may be used in
nuclear reactors instead of uranium as fuel. This produces less transuranic waste.
* Mantles in portable gas lights. These mantles glow with a dazzling light (unrelated to radioactivity) when heated in a gas flame.
* Used in
gas tungsten arc weldingelectrodes.
* Used to control the grain size of
tungstenused for electric lamps.
* Used in heat-resistant
ceramics like high-temperature laboratory crucibles.
* Added to
glass, it helps create glasses of a high refractive indexand with low dispersion. Consequently, they find application in high-quality lenses for cameras and scientific instruments.
* Has been used as a
** In the conversion of
ammoniato nitric acid.
** In producing
* Thorium dioxide is the active ingredient of
Thorotrast, which was used as part of X-raydiagnostics. This use has been abandoned due to the carcinogenicnature of Thorotrast.
M. T. Esmark found a black mineral on Løvøy Island,
Norwayand gave a sample to Professor Jens Esmark, a noted mineralogistwho was not able to identify it, so he sent a sample to the Swedish chemist Jöns Jakob Berzeliusfor examination in 1828. [cite web
accessdate=2007-01-18] Berzelius analysed it and named it after
Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the gas mantlein 1885.
Between 1900 and 1903
Ernest Rutherfordand Frederick Soddyshowed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half lifeas one of the outcomes of the alpha particleexperiments that led to their disintegration theory of radioactivity. [cite book|last=Simmons|first=John Galbraith|title=The Scientific 100|page=19|date=1996|publisher=Seacaucus NJ: Carol]
crystal bar process" (or "Iodide process") was discovered by Anton Eduard van Arkeland Jan Hendrik de Boerin 1925 to produce high-purity metallic thorium. [cite journal|last=van Arkel|first=A.E.|coauthors=de Boer, J.H.|title=Preparation of pure titanium, zirconium, hafnium, and thorium metal|journal=Zeitschrift für Anorganische und Allgemeine Chemie|volume=148|pages=345–350|date=1925]
The name ionium was given early in the study of radioactive elements to the 230Th
isotopeproduced in the decay chainof 238U before it was realized that ionium and thorium were chemically identical. The symbol Io was used for this supposed element.
Thorium is found in small amounts in most rocks and
soils, where it is about three 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, the most common being the rare-earth thorium-phosphate mineral monazite, which may contain up to about 12% thorium oxide. Thorium-containing monazite(Ce) occurs in Africa, Antarctica, Australia, Europe, India, North America, and South America. [ [http://www.mindat.org/min-2751.html Monazite-(Ce): Monazite-(Ce) mineral information and data ] ]
232Th decays very slowly (its
half-lifeis about three times the age of the earth) but other thorium isotopes occur in the thorium and uraniumdecay chains. Most of these are short-lived and hence much more radioactive than 232Th, though on a mass basis they are negligible.
"See also ."
Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand. [cite web|url=http://www.iaea.org/inis/aws/fnss/fulltext/0412_1.pdf|title=An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future|author=K.M.V. Jayaram] 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,
Australiaand Indiahave particularly large reserves of thorium. India and Australia are believed to possess approx 300,000 metric tonnes each; i.e. each country possessing 25% of the world's thorium reserves.cite web|title=US approves Indian nuclear deal|publisher=BBC News|date=2006-12-09|url=http://news.bbc.co.uk/2/hi/south_asia/6219998.stm] However, in the OECD reports, estimates of Australian's Reasonably Assured Reserves (RAR) of Thorium indicate only 19,000 metric 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 metric tonnes (USGS) and 319,000 metric tonnes (OECD/IAEA). Furthermore the IAEA report mentions that India possesses two thirds (67%) of global reserves of monazite, the primary thorium ore:
The world’s reserve of monazite is estimated to be in the range of 12 million tonnes ofwhich nearly 8 million tonnes occur with the heavy minerals in the beach sands of India in theStates of Kerala, Tamil Nadu, Andhra Pradesh and Orissa.[cite book|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|title=IAEA: Thorium fuel cycle — Potential benefits and challenges|pages=pp 45] The IAEA also states that recent reports have upgraded India's thorium deposits up from approximately 300,000 metric tonnes to 650,000 metric tonnes:
In the RAR category, the deposits in Brazil, Turkey and India are in the range of 0.60, 0.38and 0.32 million tonnes respectively. The thorium deposits in India has recently been reportedto be in the range 0.65 million tonnes.[cite book|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|title=IAEA: Thorium fuel cycle — Potential benefits and challenges|pages=pp 45] Therefore, the IAEA and OECD appear to conclude that
Braziland 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): [cite web|url=http://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs|title=U.S. Geological Survey, Mineral Commodity Summaries - Thorium] [cite web|url=http://www.world-nuclear.org/info/inf62.htm|title=Information and Issue Briefs - Thorium|publisher=World Nuclear Association|accessdate=2006-11-01] Note: The Australian figures are based on assumptions and not on actual geological surveys, therefore the figures cited for Australia may be misleading, should be treated with caution and could possibly indicate inflated values for Australia's actual reserves of thorium; note the OECD estimates of Australian's Reasonably Assured Reserves (RAR) of Thorium (listed below) indicate only 19,000 metric tonnes and not 300,000 tonnes as listed above.
* 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). [cite book|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|title=IAEA: Thorium fuel cycle — Potential benefits and challenges|pages=pp 45(table 8), 97(ref 78)]
Thorium as a nuclear fuel
Thorium, as well as
uraniumand plutonium, can be used as fuel in a nuclear reactor. Although not fissileitself, 232Th will absorb slow neutrons to produce 233U, which is fissile. Hence, like 238U, it is fertile.
Problems include the high cost of fuel fabrication due partly to the high radioactivity of 233U which is a result of its contamination with traces of the short-lived 232U; the similar problems in recycling thorium due to highly radioactive 228Th; some weapons proliferation risk 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 commercialised, and the effort required seems unlikely while (or where) abundant uranium is available.
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.
One of the earliest efforts to use a thorium fuel cycle took place at
Oak Ridge National Laboratoryin the 1960s. An experimental reactor was built based on Molten Salt Reactortechnology to study the feasibility of such an approach, using thorium- fluoridesalt kept hot enough to be liquid, thus eliminating the need for fabricating fuel elements. This effort culminated in the Molten-Salt Reactor Experimentthat used 232Th as the fertile material and 233U as the fissile fuel. Due to a lack of funding, the MSR program was discontinued in 1976.
Norwaywas debating whether or not to focus on thorium plants, due to the existence of large deposits of thorium ores in the country, particularly at Fensfeltet, near Ulefoss in Telemarkcounty.
The primary fuel of the HT3R Project near
Odessa, Texas, USAwill be ceramic-coated thorium beads.
Naturally occurring thorium is composed of one
isotope: 232Th. Twenty-seven radioisotopes have been characterized, with the most abundant and/or stable being 232Th with a half-lifeof 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 radioactiveisotopes 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 3.5 eV. [Phys. Rev. C 73 044326 (April 2006)]
The known isotopes of thorium range in
atomic weightfrom 210 u (210Th) to 236 u (236Th). [Phys. Rev. C 52, 113–116 (1995)]
Powdered thorium metal is often
pyrophoricand should be handled carefully.
Natural thorium decays very slowly compared to many other radioactive materials, and the
alpha radiationemitted cannot penetrate human skin. Owning and handling small amounts of thorium, such as a gas mantle, is considered safe if care is taken not to ingest the thorium -- lungs and other internal organs "can" be penetrated by alpha radiation. Exposure to aerosolized thorium can lead to increased risk of cancers of the lung, pancreasand blood. Exposure to thorium internally leads to increased risk of liverdiseases. This element has no known biological role. See also Thorotrast.
Thorium has been extracted chiefly from monazite through a multi-stage process. In the first stage, the monazite sand is dissolved in an inorganic acid such as sulfuric acid (H2SO4). In the second, the Thorium is extracted 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. [ Crouse, David; Brown, Keith (December 1959) " [http://pubs.acs.org/cgi-bin/abstract.cgi/iechad/1959/51/i12/f-pdf/f_ie50600a030.pdf?sessid=6006l3 The Amex Process for Extracting Thorium Ores with Alkyl Amines] "."Industrial & Engineering Chemistry" 51 (12): 1461. Retrieved on
David Hahn, who produced small quantities of fissionable material in his backyard.
Sylvania Electric Products explosion
* [http://periodic.lanl.gov/elements/90.html Los Alamos National Laboratory — Thorium]
* [http://www.webelements.com/webelements/elements/text/Th/index.html WebElements.com — Thorium]
* [http://www.uic.com.au/ The Uranium Information Centre] provided some of the original material in this article.
* [http://www.euronuclear.org/info/encyclopedia/d/decaybasinnatural.htm European Nuclear Society — Natural Decay Chains]
* [http://www.world-nuclear.org/info/inf62.htm Thorium information page]
* [http://www.cosmosmagazine.com/node/348/ New Age Nuclear: article on thorium reactors | "Cosmos Magazine"]
* [http://www.atsdr.cdc.gov/tfacts147.html ATSDR ToxFAQs — Thorium]
* [http://minerals.usgs.gov/minerals/pubs/commodity/thorium/ USGS data — Thorium]
* [http://www.orau.org/ptp/collection/quackcures/endless.htm The Endless Refrigerator/Freezer Deodorizer] , a commercial product which claimed to destroy odours 'forever.' Made with thorium-232.
* [http://news.independent.co.uk/sci_tech/article2070374.ece Is thorium the answer to our energy crisis?]
* [http://thoriumenergy.blogspot.com Thorium Energy] Blog, discussion forum and document repository
* [http://www.energyfromthorium.com/ Another thorium information page]
* [http://www.mindat.org/min-2751.html/ Monazite]
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