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{{Distinguish|Thallium|Thulium}}
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{{infobox thorium}}


'''Delete me.'''
'''Thorium''' is a naturally occurring [[radioactivity|radioactive]] [[chemical element]] with the symbol '''Th''' and [[atomic number]] 90. It was discovered in 1828 by the Norwegian mineralogist [[Morten Thrane Esmark]] and identified by the Swedish chemist [[Jons Jakob Berzelius|Jöns Jakob Berzelius]] and named after [[Thor]], the [[Norse god]] of thunder.

Thorium produces a radioactive gas, [[radon]]-220, as one of its [[decay products]]. Secondary decay products of thorium include [[radium]] and [[actinium]]. In nature, virtually all thorium is found as [[thorium-232]], which undergoes [[alpha decay]] with a [[half-life]] of about 14.05 [[1000000000 (number)|billion]] years. Other [[isotopes of thorium]] are short-lived intermediates in the decay chains of higher elements, and only found in trace amounts. Thorium is estimated to be about three to four times more abundant than [[uranium]] in the Earth's crust, and is chiefly refined from [[monazite]] sands as a by-product of extracting [[rare earth metals]].

Thorium was once commonly used as the light source in [[gas mantle]]s and as an alloying material, but these applications have declined due to concerns about its radioactivity. Thorium is also used as an alloying element in nonconsumable [[TIG]] welding electrodes. It remains popular as a material in high-end optics and scientific instrumentation; thorium and uranium are the only radioactive elements with major commercial applications that do not rely on their radioactivity.

[[Canada]], [[China]], [[Germany]], [[India]], the [[Netherlands]], the [[United Kingdom]] and the [[United States]] have experimented with using thorium as a substitute nuclear fuel in nuclear reactors.<ref name="IAEA-1450">
{{cite web
|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf
|format=PDF
|publisher=International Atomic Energy Agency
|title=IAEA-TECDOC-1450 Thorium Fuel Cycle-Potential Benefits and Challenges
|date=May 2005
|accessdate=15 March 2012
}}</ref> When compared to uranium, there is a growing interest in [[thorium-based nuclear power]] due to its greater safety benefits, absence of non-[[Fertile material|fertile]] isotopes and its higher occurrence and availability.<ref name="IAEA-1450" /> [[India's three stage nuclear power programme]] is possibly the most well known and well funded of such efforts.<ref name=NPCIL>{{cite web|last=Jain|first=S.K.|title=Nuclear Power – An alternative|url=http://www.npcil.nic.in/pdf/nuclear%20power-%20an%20alternative.pdf|publisher=NPCIL|accessdate=27 March 2012}}</ref><ref name = Argonne>{{cite news|last=Bucher|first=R.G.|title= India’s Baseline Plan for Nuclear Energy self-sufficiency stage|url=http://www.ipd.anl.gov/anlpubs/2010/05/67057.pdf|work=|publisher=Argonne National Laboratory|accessdate=27 March 2012|date= January 2009}}</ref>
{{toclimit|3}}

== Characteristics ==

=== Physical properties ===
[[File:Thorium crystal.jpg|thumb|A close view of a thorium crystal]]
Pure thorium is a silvery-white metal that 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 degree of oxide contamination greatly influences thorium's physical properties.{{sfn|Wickleder|2006|p=61}}

The purest specimens often contain several tenths of a percent of the oxide. Pure thorium is soft, very [[Ductility|ductile]], and can be [[Cold-rolling#Cold rolling|cold-rolled]], [[Swaging|swaged]], and [[Drawing (manufacturing)|drawn]]. Thorium is [[Polymorphism (materials science)|dimorphic]], changing at 1360&nbsp;°C from a [[Cubic crystal system|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.{{sfn|Wickleder|2006|p=61}}

Powdered thorium metal is often [[Pyrophoricity|pyrophoric]] and requires careful handling. When heated in air, thorium metal [[swarf|turnings]] ignite and burn brilliantly with a white light. Thorium has one of the largest liquid temperature ranges of any element, with 2946&nbsp;°C between the melting point and boiling point.<ref name=CRC/> Thorium metal is [[paramagnetic]] with a [[ground state]] of 6d<sup>2</sup>7s<sup>2</sup>.{{sfn|Wickleder|2006|p=61}}

=== Chemical properties ===
Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except [[hydrochloric acid]].<ref name=CRC/> It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride ion.<ref name="ekhyde">{{cite book| url=http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf|author = Hyde, Earl K.|title =The radiochemistry of thorium| publisher = Subcommittee on Radiochemistry, National Academy of Sciences—National Research Council| year = 1960}}</ref>

[[Thorium oxide|Thorium's oxide]] is ThO<sub>2</sub>. Thorium's most common oxidation state is +4, as in [[thorium(IV) fluoride|ThF<sub>4</sub>]], but thorium also has an oxidation state of +3, as in [[thorium(III) iodide|ThI<sub>3</sub>]]. Thorium has been shown to activate [[carbon-hydrogen bond]]s, forming unusual compounds. Thorium atoms can also bond to more atoms than any other element. For instance, in the compound [[thorium tetrakisaminodiborane]] thorium bonds to fifteen hydrogen atoms.<ref name = "Nature's Building Blocks"/>

=== Compounds ===
Thorium compounds are stable in the +4 oxidation state.<ref name="tp147-c3" />

[[Thorium dioxide]] has the highest melting point (3300&nbsp;°C) of all oxides.<ref>{{cite book|last = Emsley|first = John|title = Nature's Building Blocks|publisher = [[Oxford University Press]]|year = 2001|page = 441|isbn = 0-19-850340-7}}</ref>

Thorium(IV) nitrate and [[thorium(IV) fluoride]] are known in their hydrated forms: {{chem|Th(NO|3|)|4|·4H|2|O}} and {{chem|ThF|4|·4H|2|O}}, respectively.<ref name="tp147-c3">{{cite web| publisher = Department of Health and Human Services| title =Toxicological Profile Information Sheet| url = http://www.atsdr.cdc.gov/toxprofiles/tp147-c3.pdf| accessdate= 21 May 2009}}</ref> Thorium(IV) carbonate, {{chem|Th(CO|3|)|2|}}, is also known.<ref name="tp147-c3" />

When treated with [[potassium fluoride]] and [[hydrofluoric acid]], Th<sup>4+</sup> forms the complex anion {{chem|ThF|6|2-}}, which precipitates as an insoluble salt, {{chem|K|2|ThF|6}}.<ref name="ekhyde" />

Thorium(IV) hydroxide, {{chem|Th(OH)|4}}, is highly insoluble in water, and is not [[Amphoterism|amphoteric]]. The [[peroxide]] of thorium, ThO<sub>4</sub> or Th(O<sub>2</sub>)<sub>2</sub>, is rare in being an insoluble solid. This property can be used to separate thorium from other ions in solution.<ref name="ekhyde" />

In the presence of [[phosphate]] anions, Th<sup>4+</sup> forms precipitates of various compositions, which are insoluble in water and acid solutions.<ref name="ekhyde" />

Thorium monoxide has recently been produced through [[laser ablation]] of thorium in the presence of oxygen.<ref>{{cite journal| title =The pure rotational spectrum of the actinide-containing compound thorium monoxide | doi = 10.1039/B709343H| year =2007| last1 =Dewberry| first1 =Christopher T.| last2 =Etchison| first2 =Kerry C.| last3 =Cooke| first3 =Stephen A.| journal =Physical Chemistry Chemical Physics| volume =9| issue =35| pages =4895–7| pmid =17912418 |bibcode=2007PCCP....9.4895D}}</ref> This highly polar molecule has the largest known internal electric field.<ref>[http://www.electronedm.org/ "The ACME EDM Experiment."] electronedm.org</ref>

=== Isotopes ===
{{Main|Isotopes of thorium}}

Twenty-seven [[radioisotope]]s have been characterized, with a range in [[atomic weight]] from 210 [[atomic mass unit|u]] (<sup>210</sup>Th) to 236 u (<sup>236</sup>Th).<ref>{{cite journal| author= Uusitalo, J.| title = α decay of the new isotopes 210Th and 211Th| journal = Phys. Rev. C |volume = 52|page = 113|year =1995| doi = 10.1103/PhysRevC.52.113|bibcode = 1995PhRvC..52..113U| display-authors= 1| last2= Enqvist| first2= T.| last3= Leino| first3= M.| last4= Trzaska| first4= W.| last5= Eskola| first5= K.| last6= Armbruster| first6= P.| last7= Ninov| first7= V. }}</ref> The most stable [[isotope]]s are:
* [[Thorium-232|<sup>232</sup>Th]] with a [[half-life]] of 14.05 billion years, it represents all but a trace of naturally occurring thorium.
* <sup>230</sup>Th with a half-life of 75,380 years. Occurs as the daughter product of <sup>238</sup>U decay.
* <sup>229</sup>Th 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.<ref>{{cite journal| author= Beck|title = Energy Splitting of the Ground-State Doublet in the Nucleus 229Th| journal = Phys. Rev. Lett. |volume =98 |page =142501 |year =2007| doi =10.1103/PhysRevLett.98.142501|pmid=17501268|bibcode=2007PhRvL..98n2501B| issue= 14| author-separator= ,| author2= B. R.| display-authors= 2| last3= Beiersdorfer| first3= P.| last4= Brown| first4= G.| last5= Moody| first5= K.| last6= Wilhelmy| first6= J.| last7= Porter| first7= F.| last8= Kilbourne| first8= C.| last9= Kelley| first9= R.}}</ref>
* <sup>228</sup>Th 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 is a component of the [[magnesium]] [[alloy]] series, called [[Mag-Thor]], used in aircraft engines and rockets and imparting high [[strength of materials|strength]] and [[creep (deformation)|creep]] resistance at elevated temperatures.<ref>{{cite book|isbn = 978-0-87170-657-7|chapter = Microstructure of Magnesium and Magnesium Alloys|page =28|url = http://books.google.com/?id=0wFMfJg57YMC&pg=PA28|editor = Avedesian, Michael M.|year = 1999|publisher = ASM International|location = Materials Park, OH|title = Magnesium and magnesium alloys}}</ref><ref name=appl/> 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 in its oxide form (thoria) in [[gas tungsten arc welding]] (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.{{sfn|Wickleder|2006|p=52}} The electrodes labeled EWTH-1 contain 1% thoria, while the EWTH-2 contain 2%.<ref>{{cite book|isbn = 978-1-4018-1046-7|chapter = Types of Tungsten|page =350|url = http://books.google.com/?id=zeRiW7en7HAC&pg=RA1-PA750|author = Jeffus, Larry|year = 2003|publisher = Thomson/Delmar Learning|location = Clifton Park, N.Y.|title = Welding : principles and applications}}</ref> In electronic equipment, thorium coating of [[tungsten]] wire improves the [[electron]] [[thermionic emission|emission]] of heated [[cathode]]s.<ref name=CRC/>

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 dating|Uranium-thorium age dating]] has been used to date hominid [[fossil]]s,<ref name=CRC/> seabeds, and mountain ranges.{{sfn|Wickleder|2006|p=53}}

Environmental concerns related to radioactivity led to a sharp decrease in demand for nonnuclear uses of thorium in the 2000s.{{sfn|Wickleder|2006|p=53}}

=== Thorium compounds ===

[[File:Radioactive Lenses (group shot).jpg|thumb|right|Examples of thoriated lenses.]]

[[Thorium dioxide]] (ThO<sub>2</sub>) and thorium nitrate ({{chem|Th(NO|3|)|4}}) were used in [[Gas mantle|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.<ref name=appl/>

Thorium dioxide is a material for [[refraction (metallurgy)|heat-resistant]] [[ceramic]]s, e.g., for high-temperature laboratory [[crucible]]s.{{sfn|Wickleder|2006|p=52}} When added to [[glass]], it helps increase [[refractive index]] and decrease [[dispersion (optics)|dispersion]]. Such glass finds application in high-quality [[lens (optics)|lenses]] for cameras and scientific instruments.<ref name=CRC/> The radiation from these lenses can self-darken (yellow) them over a period of years and degrade film, but the health risks are minimal.<ref>[http://www.orau.org/ptp/collection/consumer%20products/cameralens.htm Thoriated Camera Lens (ca. 1970s)]</ref> 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]].<ref name=appl/>

Thorium dioxide has been used as a [[catalyst]] in the conversion of [[ammonia]] to [[nitric acid]],{{sfn|Wickleder|2006|p=52}} in [[petroleum]] [[cracking (chemistry)|cracking]] and in producing [[sulfuric acid]]. It is the active ingredient of [[Thorotrast]], which was used as [[radiocontrast agent]] for [[X-ray]] diagnostics. This use has been abandoned due to its [[carcinogenic]] nature.<ref name=CRC/>

Despite its radioactivity, [[Thorium(IV) fluoride|thorium fluoride]] (ThF<sub>4</sub>) is used as an antireflection material in multilayered optical coatings. It has excellent optical transparency in the range 0.35–12&nbsp;µm, and its radiation is primarily due to [[alpha particle]]s, which can be easily stopped by a thin cover layer of another material.<ref>{{cite book|url=http://books.google.com/?id=_VsEiRoFnXcC&pg=PA196|page=196|title=Optical thin films: user handbook|author=Rancourt, James D.|publisher=SPIE Press|year=1996|isbn=0-8194-2285-1}}</ref> Thorium fluoride was also used in manufacturing [[carbon arc lamp]]s, which provided high-intensity illumination for movie projectors and search lights.<ref name=appl>{{cite book|url=http://books.google.com/?id=ahNFGR1jMB4C&pg=PA81|page=81|title=Encyclopedia of Chemical Processing and Design: Thermoplastics to Trays, Separation, Useful Capacity|author=McKetta, John J. |publisher=CRC Press|year=1996|isbn=0-8247-2609-X}}</ref>

=== Thorium as a nuclear fuel ===
{{see also|Thorium-based nuclear power}}

==== Benefits and challenges ====
The naturally occurring isotope thorium-232 is a [[fertile material]], and with a suitable neutron source can be used as [[nuclear fuel]] in [[nuclear reactors]], including [[breeder reactor]]s. <!-- BETTER SOURCES NEEDED A thorium fuel cycle offers several potential advantages over a [[uranium fuel cycle]] including much [[Thorium#Occurrence|greater abundance]] on Earth, superior physical and nuclear properties of the fuel, enhanced [[nuclear proliferation|proliferation]] resistance, and reduced nuclear waste production.<ref name=Wired>[http://www.wired.com/magazine/2009/12/ff_new_nukes/ "Uranium is So Last Century — Enter Thorium, the New Green Nuke"], ''Wired'' magazine, 21 Dec 2009</ref> Nobel laureate [[Carlo Rubbia]] at [[CERN]] (European Organization for Nuclear Research) states that a tonne of thorium can produce as much energy as 200 tonnes of uranium, or 3,500,000 tonnes of coal.<ref name=Pritchard>[http://www.telegraph.co.uk/finance/comment/7970619/Obama-could-kill-fossil-fuels-overnight-with-a-nuclear-dash-for-thorium.html "Obama could kill fossil fuels overnight with a nuclear dash for thorium"], Evans-Pritchard, Ambrose. ''The Telegraph'', U.K. 29 August 2010</ref><ref>[http://www.bbc.co.uk/news/science-environment-13040853 "Using thorium could reduce risk of nuclear power"], ''BBC'', 12 April 2011, audio interview with Carlo Rubbia</ref> One of the early pioneers of the technology was U.S. physicist [[Alvin Weinberg]] at [[Oak Ridge National Laboratory]] in Tennessee, who helped develop a working nuclear plant using liquid fuel in the 1960s. --> In 1997, the U.S. Energy Department underwrote research into thorium fuel, and research also was 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.<ref name=Radkowsky>[http://books.google.com/books?id=2wsAAAAAMBAJ&pg=PA19 ''Bulletin of the Atomic Scientists'']. September/October 1997 pp. 19–20</ref>

Radkowsky was chief scientist in the U.S. [[nuclear submarine]] program directed by [[Admiral Hyman Rickover]] and later headed the design team that built the USA's first civilian [[Shippingport Atomic Power Station|nuclear power plant at Shippingport, Pennsylvania]], which was a scaled-up version of the first naval reactor.<ref name=Radkowsky/> The third Shippingport core, initiated in 1977, bred thorium.<ref name="asme-landmark">{{cite web |title= Historic Achievement Recognized: Shippingport Atomic Power Station, A National Engineering Historical Landmark |url=http://files.asme.org/ASMEORG/Communities/History/Landmarks/5643.pdf |format=PDF |pages=4 |accessdate=24 June 2006}}</ref> Even earlier [[Thorium fuel cycle#List of thorium-fueled reactors|examples]] of reactors using fuel with thorium exist, including the first core at the [[Indian Point Energy Center]] in 1962.<ref>{{cite web|url=http://www.americanscientist.org/issues/feature/thorium-fuel-for-nuclear-energy/2|title=Thorium Fuel for Nuclear Energy|publisher=American Scientist|date=Sep/October 2003}}</ref>

Some countries, including [[India]], are now investing in research to build thorium-based nuclear reactors. <!-- BETTER SOURCES NEEDED Anil Kakodkar, chairman of the Indian Atomic Energy Commission, said in 2009 that his country has a "long-term objective goal of becoming energy-independent based on its vast thorium resources."<ref name=nyt>[http://www.nytimes.com/2009/10/20/business/global/20renthorium.html?pagewanted=1&sq=Thorium%20reactor&st=cse&scp=1 "Considering an Alternative Fuel for Nuclear Energy"], ''New York Times,'' 19 October 2009</ref><ref>[http://www.youtube.com/watch?v=Nl5DiTPw3dk&feature=player_embedded "India's experimental Thorium Fuel Cycle Nuclear Reactor [NDTV Report]", Video report, 2010, 7 minutes</ref> In May 2010, researchers from [[Ben-Gurion University]] in Israel and [[Brookhaven National Laboratory]] in New York, received a grant to develop a thorium-based, self-sustaining light water reactor<ref name=Israel/> that produces and consumes about the same amounts of fuel.<ref name=Israel>[http://www.israel21c.org/201010118407/environment/self-sustaining-nuclear-energy-from-israel "Self-sustaining nuclear energy from Israel"] Israel21c News Service, 11 October 2010</ref> In the U.S., NASA scientist and thorium expert Kirk Sorensen calls it the "next giant leap" in energy technology, noting that the "potential energy in thorium is staggering," explaining how just 8 tablespoons of thorium could provide the energy used by an American during his or her lifetime.<ref>[http://www.youtube.com/watch?v=AZR0UKxNPh8&feature=player_embedded#at=219 "Energy From Thorium"], talk at Google Tech Talks, 23 July 2009, video, 1 hr. 22 min.</ref><ref>[http://www.technewsdaily.com/nuclear-power-thorium-future-2400/ "Nuclear Power's Next Fuel Is a Blast from the Past"] ''TechNews Daily'', 24 March 2011</ref>
-->
A 2005 report by the [[International Atomic Energy Agency]] discusses potential benefits along with the challenges of thorium reactors.<ref>[http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf "Thorium fuel cycle — Potential benefits and challenge"], IAEA, May 2005</ref> <!-- BETTER SOURCES NEEDED According to Australian science writer Tim Dean, "thorium promises what uranium never delivered: abundant, safe, and clean energy – and a way to burn up old radioactive waste."<ref name=Dean>[http://www.cosmosmagazine.com/features/print/348/new-age-nuclear?page=0,0 "New Age Nuclear"] ''Cosmos Magazine'', April 2006</ref> With a thorium nuclear reactor, Dean stresses a number of added benefits: there is no possibility of a meltdown, it generates power inexpensively, it does not produce weapons-grade by-products, and burns up existing high-level waste and nuclear weapon stockpiles.<ref name=Dean/> [[Ambrose Evans-Pritchard]], of the British ''Daily Telegraph'', suggests that "Obama could kill fossil fuels overnight with a nuclear dash for thorium," and could put "an end to our dependence on fossil fuels within three to five years."<ref name=Pritchard/> He also points out that "[[China]] is leading the way" with its own "dash for thorium," which it announced in March 2011.<ref>[http://www.telegraph.co.uk/finance/comment/ambroseevans_pritchard/8393984/Safe-nuclear-does-exist-and-China-is-leading-the-way-with-thorium.html "Safe nuclear does exist, and China is leading the way with thorium"], ''The Telegraph'', U.K. 20 March 2011</ref> --> [[India]] has also made thorium-based nuclear reactors a priority with its focus on developing [[fast breeder]] technology.<ref>[http://nextbigfuture.com/2008/08/indias-thorium-nuclear-reactor-and.html Progress on India's Thorium Nuclear Reactor and South Africa's Pebble Bed]. Nextbigfuture.com (22 August 2008). Retrieved on 2011-05-01.</ref><ref>[http://www.world-nuclear.org/info/inf53.html Nuclear Power in India|Indian Nuclear Energy]. World-nuclear.org. Retrieved on 1 May 2011.</ref>

Some benefits of thorium fuel when compared with uranium were summarized as follows:<ref>{{cite book|author=Ayhan Demirbas|title=Biohydrogen: for future engine fuel demands|url=http://books.google.com/books?id=1w9DMBh7JOUC&pg=PA38|year= 2009|publisher=Springer|isbn=978-1-84882-510-9|pages=36–39}}</ref><!--bulleted list in original source-->
<blockquote>
* Weapons-grade fissionable material (<sup>233</sup>U) is harder to retrieve safely and clandestinely from a thorium reactor;
* Thorium mining produces a single pure isotope, whereas the mixture of natural uranium isotopes must be enriched to function in most common reactor designs. 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,<ref>[http://www.cavendishscience.org/bks/nuc/thrupdat.htm "Thorium: Is It the Better Nuclear Fuel?"], Cavendish Press, Dec 2008</ref> so fission stops by default in an accelerator driven reactor.
</blockquote>

When used in a breeder-like reactor, however, unlike uranium-based light water reactors, thorium requires irradiation and reprocessing before the above-noted advantages of thorium-232 can be realized, which initially makes solid thorium fuels more expensive than uranium fuels.{{sfn|Wickleder|2006|p=53}} 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.<ref>{{cite book|author=Kursunoglu, Behram N., and Teller, Edward|title=Global warming and energy policy|url=http://books.google.com/books?id=7ndrQgAACAAJ|year= 2001|publisher=Kluwer Academic/Plenum Publishers|isbn=978-0-306-46635-9|page=4}}</ref>

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 10,000 years."<ref name=TEA>[http://thoriumenergyalliance.com/ThoriumSite/portal.html What Is Thorium?] Thorium Energy Alliance</ref>

==== Thorium energy fuel cycle ====
{{Main|Thorium fuel cycle}}

{{Quote box
|quote = ''"Thorium is like wet wood […it] needs to be turned into fissile uranium just as wet wood needs to be [[wood drying|dried]] in a furnace."''
|source = — [[Ratan Kumar Sinha]], current Chairman of the [[Atomic Energy Commission of India]].<ref>{{cite news |title=Date set for fuel reactor |url=http://www.telegraphindia.com/1130903/jsp/nation/story_17305278.jsp |newspaper=[[The Telegraph (Calcutta)]] |date=2 September 2013 |accessdate=4 September 2013}}</ref>
|width = 220px
}}

Like [[Uranium-238|<sup>238</sup>U]], <sup>232</sup>Th is not [[fissile]] itself, but it is [[Fertile material|fertile]]: it absorbs [[slow neutron]]s to produce, after two [[beta decay]]s, [[uranium-233|<sup>233</sup>U]], which is fissile.{{sfn|Wickleder|2006|p=53}} The preparation of thorium fuel does not require [[isotopic separation]], unlike the preparation of uranium fuels.

The [[thorium fuel cycle]] creates [[uranium-233|<sup>233</sup>U]], which, if separated from the reactor's fuel, could with some difficulty be used for making nuclear weapons. This is one reason why a liquid-fuel cycle (e.g., [[molten salt reactor]] or MSR) is preferred—only a limited amount of <sup>233</sup>U ever exists in the reactor and its heat-transfer systems, preventing access to weapons material. However, the neutrons produced by the reactor can be absorbed by a thorium or uranium blanket to produce fissile [[uranium-233|<sup>233</sup>U]] or [[plutonium-239|<sup>239</sup>Pu]]. Also, the [[uranium-233|<sup>233</sup>U]] could be continuously [[liquid-liquid extraction|extracted]] from the molten fuel as the reactor runs. Neutrons from the decay of uranium-233 can be fed back into the fuel cycle to start the cycle again.{{sfn|Wickleder|2006|p=53}}

The neutron flux from spontaneous fission of <sup>233</sup>U is negligible. <sup>233</sup>U can thus be used easily in a simple gun-type nuclear bomb design.<ref>{{cite journal| url= http://phys4.harvard.edu/~wilson/publications/ppaper703.html| title = Accelerator Driven Subcritical Assemblies| journal = Report to Energy Environment and Economy Committee, U.S. Global Strategy Council| author = Wilson, R.| year = 1998}}</ref> In 1977, a light-water reactor at the [[Shippingport Atomic Power Station]] was used to establish a <sup>232</sup>Th-<sup>233</sup>U fuel cycle. The reactor worked until its decommissioning in 1982.<ref>{{cite web |url=http://www.atomicinsights.com/oct95/LWBR_oct95.html |title=Light Water Breeder Reactor:Adapting a Proven System |publisher=AtomicInsights.com |accessdate=3 September 2009}}</ref><ref>{{cite web |url=http://www.world-nuclear.org/info/inf62.html |title=Thorium |publisher=World Nuclear Association|accessdate=3 September 2009}}</ref><ref>{{cite web|title = The Shippingport Pressurized Water Reactor and Light Water Breeder Reactor|last = Clayton |first = J.C.| year = 1993|publisher = 25. American Chemical Society meeting, Pittsburgh, PA|url = http://www.osti.gov/energycitations/servlets/purl/10191380-PWWUeT/native/10191380.pdf}}</ref> 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),<ref>Sollychin, Ray. (3 September 2009) [http://www.iaea.org/Publications/Magazines/Bulletin/Bull511/51104894344.html Exploring Fuel Alternatives]. Iaea.org. Retrieved on 2011-05-01.</ref> conducted by the [[International Atomic Energy Agency]] (IAEA).
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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|date=January 2010}} with the 2009 decision by the US to shelve the [[Yucca Mountain nuclear waste repository]] highlighting the issue of what to do with all the plutonium left over from decommissioned nuclear weapons.<ref>{{cite web|url=http://www.thoriumpower.com/files/VPI_03091a.pdf|title=Thorium Will Power Your Profits (and Reduce Terror Risks)|author=Breakthrough Technology Alert|date=March 2009}}</ref> Thorium Power, with offices in London, Dubai, and Moscow and with [[Hans Blix|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 [[Indo-US civilian nuclear agreement|1-2-3 Agreement]] to complete a joint-venture with [[Punj Lloyd]], an Indian engineering firm with nuclear reactor construction ambitions.<ref>{{cite news|url=http://www.forbes.com/2008/12/09/thorium-power-india-markets-equity-cx_vr_1209markets07.html|title=Thorium Warms Up To Nuclear India|author=Forbes.com|date=9 December 2008}}</ref> -->

Unlike its use in [[Molten Salt Reactor|Molten salt reactors]], when using solid thorium in modified [[Light water reactor|light water reactor (LWR)]] problems include: the undeveloped technology for fuel fabrication; in traditional, once-through [[Light water reactor|LWR]] designs potential problems in recycling thorium due to highly radioactive <sup>228</sup>Th; some weapons proliferation risk due to production of <sup>233</sup>U; 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 [[Light water reactor|LWR]]. The effort required has not seemed worth it while abundant uranium is available.
<!-- WRONG SECTION - THIS IS HISTORY 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.<ref>{{cite web|url=http://hatch.senate.gov/public/index.cfm?FuseAction=PressReleases.Detail&PressRelease_id=bf8479e9-1b78-be3e-e031-1635aee6efa9|title=Sens. hatch and reid push for thorium nuclear fuel cycle|author=Senator Orrin Hatch website}}</ref> The bill, however, did not reach a full Senate vote. -->
<!-- REDUNDANT AND NEEDS CITE
The [[thorium fuel cycle]], with its potential for breeding fuel without [[fast neutron reactor]]s, holds considerable potential long-term benefits. Perhaps more importantly, thorium produces one to two orders of magnitude less long-lived [[transuranic]]s than uranium fuel cycles, though the long-lived [[actinide]] [[protactinium-231]] is produced, and the amount of [[fission product]]s is similar. -->
<!-- WRONG SECTION - THIS IS HISTORY
An early effort to use a thorium fuel cycle took place at [[Oak Ridge National Laboratory]] (ORNL) in the 1960s. An experimental reactor was built based on [[molten salt reactor|MSR]] technology to study the feasibility of such an approach, using thorium-[[fluoride]] [[Salt (chemistry)|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 <sup>232</sup>Th as the fertile material and <sup>233</sup>U 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 thought of as a [[Generation IV reactor]].-->

==== Commercial nuclear power station ====
[[India]]'s [[Kakrapar Atomic Power Station|Kakrapar-1]] reactor is the world's first reactor that uses thorium rather than depleted uranium for power flattening across the reactor core.<ref>{{cite web|url=http://www.power-technology.com/features/feature1141/ |title=Thorium: Cleaner Nuclear Power?|date=10 August 2007 |publisher=Powertechnology.com}}</ref> 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 2016,<ref>{{cite web |url =http://ibnlive.in.com/news/construction-of-7th-rajasthan-nuclearplant-begins/424450-62-128.html | title = Construction of 7th Rajasthan nuclear-plant begins | date= Sep 25, 2013 | publisher = IBN | accessdate=2013-10-26}}</ref> after which they plan to construct five more reactors.<ref>{{cite news|url=http://timesofindia.indiatimes.com/articleshow/3864684.cms|title=Development work on 300 MW advanced heavy water reactor at advanced stage|work=The Times Of India|date=20 December 2008}}</ref><ref>{{cite news|url=http://www.thehindubusinessline.com/economy/work-on-thoriumbased-reactor-to-commence-soon/article2916878.ece|title=Work on thorium-based reactor to commence soon |work=The Hindu|date=21 February 2012}}</ref> The 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.<ref name=nyt>{{cite news|url=http://www.nytimes.com/2009/10/20/business/global/20renthorium.html?pagewanted=1&sq=Thorium%20reactor&st=cse&scp=1 |title=Considering an Alternative Fuel for Nuclear Energy|newspaper=New York Times |date= 19 October 2009|first=Lisa|last=Pham}}</ref> India currently envisages meeting 30% of its electricity demand through thorium-based reactors by 2050.<ref>{{cite web|url=http://www.indiadaily.com/editorial/19093.asp |title=Indian Thorium based reactor design complete}}</ref>

==== Existing thorium energy projects ====
{{Main|Thorium-based nuclear power#Current thorium projects}}

While research is under way in many countries, only India is building utility-scale plants, mostly planned to thorium-fueled. In 2012, the first commercial fast reactor capable of using thorium was allegedly nearing completion, according to Srikumar Banerjee, former Chairman of the Indian Atomic Energy Commission.<ref>[http://www.thehindu.com/todays-paper/tp-national/article3582922.ece "First commercial fast reactor nearly ready"], ''The Hindu'', June 29, 2012</ref>

==== 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 fissioned.

== History ==
[[File:Keplers supernova.jpg|thumb|right|The Earth's thorium originated in the [[wikt:death throe|death throes]] of ancient stars.]]
[[Morten Thrane Esmark]] found a black mineral on [[Løvøya, Telemark|Løvøya]] 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 the Swedish chemist [[Jöns Jakob Berzelius]] for examination in 1828. Berzelius determined that it contained a new element, which he named thorium after [[Thor]], the [[norse mythology|Norse god]] of thunder.{{sfn|Wickleder|2006|p=52}} He published his findings in 1829.<ref name="Weeks"/><ref>{{cite journal|author=Berzelius, J. J. |year=1829|url=http://gallica.bnf.fr/ark:/12148/bpt6k151010.pleinepage.r=Annalen+der+Physic.f395.langFR|title=Untersuchung eines neues Minerals und einer darin erhalten zuvor unbekannten Erde (Investigation of a new mineral and of a previously unknown earth contained therein)|journal=Annalen der Physik und Chemie|volume= 16| pages =385–415|doi=10.1002/andp.18290920702|bibcode=1829AnP....92..385B|issue=7}} (modern citation: ''Annalen der Physik'', vol. 92, no. 7, pp. 385–415)</ref><ref>{{cite journal|author= Berzelius, J. J. |year= 1829|title=Undersökning af ett nytt mineral (Thorit), som innehåller en förut obekant jord" (Investigation of a new mineral (thorite), as contained in a previously unknown earth)|journal=Kungliga Svenska Vetenskaps Akademiens Handlingar (Transactions of the Royal Swedish Science Academy)| pages=1–30}}</ref> Berzelius reused the name of a previous element discovery from a mineral from the [[Falun]], which later proved to be a yttrium mineral.<ref name="Weeks">{{cite journal | doi = 10.1021/ed009p1231|bibcode = 1932JChEd...9.1231W | title = The discovery of the elements. XI. Some elements isolated with the aid of potassium and sodium: Zirconium, titanium, cerium, and thorium | year = 1932 | last1 = Weeks | first1 = Mary Elvira |authorlink1=Mary Elvira Weeks| journal = Journal of Chemical Education | volume = 9 | issue = 7 | pages = 1231 }}</ref><ref>{{cite journal | doi = 10.1002/ange.19020153703 | title = Die eigentlichen Thorit-Mineralien (Thorit und Orangit) | year = 1902 | last1 = Schilling | first1 = Johannes | journal = Zeitschrift für Angewandte Chemie | volume = 15 | issue = 37 | pages = 921}}</ref> The metal had no practical uses until [[Carl Auer von Welsbach]] invented the [[gas mantle]] in 1885.{{sfn|Wickleder|2006|p=52}}

Thorium was first observed to be radioactive in 1898, independently, by the Polish-French physicist [[Marie Curie]] and the German chemist [[Gerhard Carl Schmidt]].<ref>{{cite journal|author=Curie, Marie |year=1898|title= Rayons émis par les composés de l'uranium et du thorium (Rays emitted by compounds of uranium and thorium)|journal=Comptes Rendus|volume =126| pages =1101–1103|ol=24166254M }}</ref><ref>{{cite journal|author=Schmidt, G. C. |year=1898|title=Über die vom Thorium und den Thoriumverbindungen ausgehende Strahlung (On the radiation emitted by thorium and thorium compounds) |journal=Verhandlungen der Physikalischen Gesellschaft zu Berlin (Proceedings of the Physical Society in Berlin)| volume= 17| pages= 14–16}}</ref><ref>{{cite journal|author=Schmidt, G. C. |url=http://gallica.bnf.fr/ark:/12148/bpt6k153068.image.r=Annalen+der+Physic.f149.langFR |title=Über die von den Thorverbindungen und einigen anderen Substanzen ausgehende Strahlung (On the radiation emitted by thorium compounds and some other substances) |journal=Annalen der Physik und Chemie |volume= 65| pages =141–151|year= 1898}} (modern citation: ''Annalen der Physik'', vol. 301, pages 141–151 (1898)).</ref> 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]].<ref>{{cite book|last=Simmons|first=John Galbraith|title=The Scientific 100: A Ranking of the Most Influential Scientists, Past and Present|page=19|year=1996|publisher=Seacaucus NJ: Carol|isbn=0806521392}}</ref>

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.<ref>{{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|year=1925|doi=10.1002/zaac.19251480133}}</ref>

The name ionium was given early in the study of radioactive elements to the <sup>230</sup>Th [[isotope]] produced in the [[decay chain]] of [[Uranium-238|<sup>238</sup>U]] before it was realized that ionium and thorium were chemically identical. The symbol '''Io''' was used for this supposed element.

[[File:Evolution of Earth's radiogenic heat.jpg|thumb|right|The [[radiogenic heat]] from the decay of <sup>232</sup>Th is a major contributor to the [[earth's internal heat budget]].]]

Thorium-232 is a [[primordial nuclide]], having existed in its current form for [[Age of the Earth|over 4.5 billion years]], predating the [[formation of the Earth]]; it was forged in the cores of dying stars through the [[r-process]] and scattered across the galaxy by [[supernova]]s.<ref>[http://www.gsi.de/forschung/kp/kp2/nuc-astro/HeavyElements_e.html Synthesis of heavy elements]</ref> Its [[radioactive decay]] produces a significant amount of the [[Earth#Heat|Earth's internal heat]].<ref name=NGJuly11>{{cite doi|10.1038/ngeo1205}}</ref>

== Occurrence ==

{{Category see also|Thorium minerals}}

[[File:NAMrad Th let.gif|thumb|Partial North American map of thorium concentrations from the [[United States Geological Survey]].]]

[[File:Lunar KREEP concentrations.jpg|thumb|Thorium concentrations on the [[moon]], as mapped by [[Lunar Prospector]].]]

Thorium is found in small amounts in most rocks and [[soil]]s; it is three times more abundant than [[tin]] in the Earth's crust and is about as common as [[lead]].{{sfn|Wickleder|2006|p=55}} Soil commonly contains an average of around 6 parts per million (ppm) of thorium.<ref>[http://www.atsdr.cdc.gov/tfacts147.pdf THORIUM] [[Agency for Toxic Substances and Disease Registry]]. July 1999.</ref> Thorium occurs in several [[mineral]]s including [[thorite]] (ThSiO<sub>4</sub>), [[thorianite]] (ThO<sub>2</sub> + UO<sub>2</sub>) and [[monazite]]. Thorianite is a rare mineral and may contain up to about 12% thorium oxide. [[Monazite]] contains 2.5% thorium, [[allanite]] has 0.1 to 2% thorium and [[zircon]] can have up to 0.4% thorium.{{sfn|Wickleder|2006|p=56}} Thorium-containing minerals occur on all continents.<ref name=CRC>{{cite book| author = Hammond, C. R. |title = The Elements, in Handbook of Chemistry and Physics 81st edition| publisher =CRC press| isbn = 0-8493-0485-7| year = 2004}}</ref><ref>{{cite web| url =http://www.mindat.org/min-2751.html |title = Monazite-(Ce): Monazite-(Ce) mineral information and data|accessdate = 18 May 2009}}</ref><ref name=usgssumm /> 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.{{sfn|Wickleder|2006|p=53}}

Thorium concentrations near the surface of the earth can be mapped using [[gamma spectroscopy]]. The same technique has been used to detect concentrations on the surface of the moon; the [[near side of the Moon|near side]] has high abundances of relatively Thorium-rich [[KREEP]], while the [[Compton–Belkovich Thorium Anomaly]] was detected on the [[far side of the Moon|far side]]. [[mars|Martian]] thorium has also been mapped by [[2001 Mars Odyssey]].<ref>[http://grs.lpl.arizona.edu/latestresults.jsp?lrid=32 Lunar &amp; Planetary Lab at The University of Arizona] January 2008: Thorium Map</ref>

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

== Extraction ==
{{Main|Monazite}}
[[Image:Monazit opening acid.gif|center|700px]]
[[File:MonaziteUSGOV.jpg|thumb|Monazite, a rare earth and thorium phosphate mineral, is the primary source of the world's thorium.]]
Thorium has been extracted chiefly from monazite through a complex multi-stage process. The monazite sand is dissolved in hot concentrated [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>). 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.<ref>{{cite journal| doi =10.1021/ie50600a030| title =The Amex Process for Extracting Thorium Ores with Alkyl Amines| year =1959| author =Crouse, David| journal =Industrial & Engineering Chemistry| volume =51| page =1461| last2 =Brown| first2 =Keith| issue =12}}</ref>

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.<ref name=CRC/>

== Reserve estimates ==

[[File:India-locator-map-thorium2012.svg|thumb|right|India's thorium is mostly found in a [[contiguous]] belt formed by its eastern coastal states.<BR />
2012 reserve estimates:<ref name="rsus3721">{{cite web |url=http://www.dae.nic.in/writereaddata/rsus3721.pdf |title=DETAILS OF THORIUM RESERVES |date=10 May 2012 |website=[[Department of Atomic Energy]] (India) |accessdate=12 December 2013}}</ref>
{{legend|#2B0000|35% ([[Andhra Pradesh]])}}
{{legend|#800000|15-20% ([[Tamil Nadu]], [[Odisha]])}}
{{legend|#D40000|10-15% ([[Kerala]], [[West Bengal]])}}
{{legend|#FF2A2A|0-5% ([[Bihar]])}}]]

Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant [[demand]].<ref>{{cite web|url= http://web.archive.org/web/20110628234922/http://www.iaea.org/inisnkm/nkm/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=Jayaram, K.M.V.}}</ref> 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.

India and Australia are believed to possess about 300,000 [[tonne]]s each; i.e., each has 25% of the world's thorium reserves.<ref name="bbc">{{cite news|title=US approves Indian nuclear deal|publisher=BBC News|date=9 December 2006|url=http://news.bbc.co.uk/2/hi/south_asia/6219998.stm}}</ref> In the OECD reports, however, 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).

Both the IAEA and OECD appear to conclude that [[India]] may possess the lion's share of world's thorium deposits.

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.<ref name=fuel>{{cite book|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|title=IAEA: Thorium fuel cycle — Potential benefits and challenges|page=45}}</ref> A government of India estimate, shared in the country's Parliament in August 2011, puts the recoverable reserve at 846,477 tonnes.<ref name=pib>{{cite news|last=|first=|title=Availability of Thorium |url=http://pib.nic.in/newsite/erelease.aspx?relid=74293|work=|publisher=Press Information Bureau, Government of India|accessdate=27 March 2012|date=10 August 2011}}</ref> The Indian Minister of State [[V. Narayanasamy]] stated that as of May 2013, the country's thorium reserves were 11.93 million tonnes (monazite, having 9-10% ThO<sub>2</sub><ref name="rsus3721" />), with a significant majority (8.59 Mt; 72%) found in the three eastern coastal states of Andhra Pradesh (3.72 Mt; 31%), Tamil Nadu (2.46 Mt; 21%) and Odisha (2.41 Mt; 20%).<ref>{{cite news |title=Over 1.25 MT thorium reserve found in 3 years |author=[[Press Trust of India]] |url=http://www.business-standard.com/article/pti-stories/over-1-25-mt-thorium-reserve-found-in-3-years-113081400960_1.html |newspaper=[[Business Standard]] |date=14 August 2013 |accessdate=22 August 2013}}</ref>

The prevailing estimate of the economically available thorium reserves comes from the U.S. Geological Survey, Mineral Commodity Summaries (1996–2010):<ref name=usgssumm>{{cite web|url=http://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs|title=U.S. Geological Survey, Mineral Commodity Summaries – Thorium}}</ref><ref>{{cite web|url=http://www.world-nuclear.org/info/inf62.htm|title=Information and Issue Briefs – Thorium|publisher=World Nuclear Association|accessdate=1 November 2006}}</ref>
{|class="wikitable" style="display: inline-block;"
|+ [[USGS]] Estimates in [[tonne]]s (1999)<ref>{{cite web |url=http://www.aesj.or.jp/~recycle/gl2005pr1-4mr.sokolov.pdf |title=Status of Nuclear Power: A Global View. Y.A Sokolov 2005 IAEA}}</ref>
|-
! align="left"| Country
! Reserves
|-
|align="left"|Australia
| 300,000
|-
| align="left"| India
| 290,000
|-
| align="left"|Norway
| 170,000
|-
| align="left"| United States
| 160,000
|-
| align="left"| Canada
| 100,000
|-
| align="left"| South Africa
| 35,000
|-
| align="left"| Brazil
| 16,000
|-
| align="left"| ''Other Countries''
| 95,000
|-
| align="center"| ''World Total''
| 1,200,000
|}

{|class="wikitable" style="display: inline-block;"
|+ [[USGS]] Estimates in [[tonne]]s (2011)
|-
! align="left"| Country
! Reserves
|-
| align="left"| India
| 963,000
|-
| align="left"| United States
| 440,000
|-
| align="left"| Australia
| 300,000
|-
| align="left"| Brazil
| 16,000
|-
| align="left"| Canada
| 100,000
|-
| align="left"| Malaysia
| 4,500
|-
| align="left"| South Africa
| 35,000
|-
| align="left"| ''Other Countries''
| 90,000
|-
| align="center"| ''World Total''
| 1,913,000
|}

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.<ref name="AIMR 2009">{{cite web|url=http://www.australianminesatlas.gov.au/aimr/commodity/thorium_09.jsp#world_ranking|title=Thorium – AIMR 2009|publisher=Australian Mines Atlas|accessdate=1 June 2009}}</ref> Adding to the confusion are subjective claims made by the Australian government (in 2009, through [[Geoscience Australia|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,<ref name="AIMR 2009"/> 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):<ref>{{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=45(table 8), 97(ref 78)}}</ref>
{| class="sortable wikitable" style="text-align:right;"
|+ [[International Atomic Energy Agency|IAEA]] Estimates in [[tonne]]s (2005)
|-
! align="left"| Country
! RAR Th
! EAR Th
|-
| align="left"| India
| 519,000
| 21%
|-
| align="left"| Australia
| 489,000
| 19%
|-
| align="left"| USA
| 400,000
| 13%
|-
| align="left"| Turkey
| 344,000
| 11%
|-
| align="left"| Venezuela
| 302,000
| 10%
|-
| align="left"| Brazil
| 302,000
| 10%
|-
| align="left"| Norway
| 132,000
| 4%
|-
| align="left"| Egypt
| 100,000
| 3%
|-
| align="left"| Russia
| 75,000
| 2%
|-
| align="left"| Greenland
| 54,000
| 2%
|-
| align="left"| Canada
| 44,000
| 2%
|-
| align="left"| South Africa
| 18,000
| 1%
|-
| align="left"| "Other countries"
| 33,000
| 2%
|-
| align="center"| "World total"
| 2,810,000
|
|-
|}

The preceding reserve figures refer to the amount of thorium in high-concentration deposits inventoried so far and estimated to be extractable at current market prices; millions of times more total exist in Earth's 3{{e|19}} tonne crust, around 120 trillion tons of thorium, and lesser but vast quantities of thorium exist at intermediate concentrations.<ref>Ragheb, M. (12 August 2011) [http://www.scribd.com/doc/105448071/Thorium-Resources-in-Rare-Earth-Elements-Ragheb-M-Aug-2011 Thorium Resources In Rare Earth Elements]. scribd.com</ref><ref>American Geophysical Union, Fall Meeting 2007, abstract #V33A-1161. [http://adsabs.harvard.edu/abs/2007AGUFM.V33A1161P Mass and Composition of the Continental Crust]</ref><ref name = reserve_economics>James D. Gwartney, Richard L. Stroup, Russell S. Sobel, David MacPherson. ''Economics: Private and Public Choice, 12th Edition''. South-Western Cengage Learning, [http://books.google.com/books?id=yIbH4R77OtMC&pg=PA730 p. 730]</ref> Proved reserves are "a poor indicator of the total future supply of a mineral resource."<ref name = reserve_economics/>

The [[Lemhi Pass]], along the [[Idaho]]-[[Montana]] border, has one of the world's largest known high quality thorium deposits. Thorium Energy, Inc. has the [[mineral rights]] to approximately 1360 [[acre]]s (5.5 [[sq km]]) of it and states that they have proven thorium oxide reserves of 600 thousand tons and probable reserves of an additional 1.8 million tons within their claim.<ref>[http://www.thoriumenergy.com/index.php?option=com_content&task=view&id=17&Itemid=33 Lemhi Pass Thorium]. thoriumenergy.com</ref>

In event of a [[thorium fuel cycle]], [[Conway granite]] with 56 (±6) [[parts per million]] thorium could provide a major low-grade resource; a 307 sq mile (795 sq km) "main mass" in [[New Hampshire]] is estimated to contain over three million metric tons per 100 feet (30 m) of depth ([[i.e.]] 1&nbsp;kg thorium in eight [[cubic metre]]s of rock), of which two-thirds is "readily [[Leaching (chemistry)|leachable]]".<ref>{{cite journal|pmid=16591014|pmc=221093|year=1962|last1=Adams|first1=JA|last2=Kline|first2=MC|last3=Richardson|first3=KA|last4=Rogers|first4=JJ|title=The Conway Granite of New Hampshire As a Major Low-Grade Thorium Resource|volume=48|issue=11|pages=1898–905|journal=Proceedings of the National Academy of Sciences of the United States of America|bibcode=1962PNAS...48.1898A|doi=10.1073/pnas.48.11.1898}}</ref> Even common granite rock with 13 PPM thorium concentration (just twice the crustal average, along with 4 ppm uranium) contains potential nuclear energy equivalent to 50 times the entire rock's mass in coal,<ref>Hubbert, M. King [http://www.energybulletin.net/node/13630 Nuclear Energy and the Fossil Fuels]. American Petroleum Institute Conference, 8 March 1956. Republished on 8 March 2006, by the Energy Bulletin.</ref> although there is no incentive to resort to such very low-grade deposits so long as much higher-grade deposits remain available and cheaper to extract.<ref>Brown, Harrison (1954). ''The Challenge of Man's Future''. New York: Viking Press.</ref> Thorium has been produced in excess of demand from the refining of [[rare earth element]]s.<ref>Hedrick, James B. (1997) [http://minerals.usgs.gov/minerals/pubs/commodity/thorium/690497.pdf Thorium]. US Geological Survey.</ref>

== Dangers and biological roles ==
[[File:PSM V74 D233 Thorium radioactive incandescent gas mantle placed above plant seeds.png|thumb|right|Experiment on the effect of radiation (from an unburned thorium gas mantle) on the germination and growth of [[timothy-grass]] seed; from ''[[Popular Science]]'', 1909.]]

Powdered thorium metal is [[pyrophoric]] and often ignites 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, however, can lead to increased risk of [[cancer]]s of the [[lung]], [[pancreas]], and [[blood]],{{citation needed|date=August 2012}} as lungs and other internal organs can be penetrated by alpha radiation. Exposure to thorium internally leads to increased risk of [[liver]] diseases. Thorium is radioactive and produces a radioactive gas, [[radon]]-220, as one of its [[decay products]]. Secondary decay products of thorium include [[radium]] and [[actinium]]. Because of this, there are concerns about the safety of thorium mantles. Some [[nuclear safety]] agencies make recommendations about their use.<ref>[http://web.archive.org/web/20071014211034/http://arpansa.gov.au/RadiationProtection/Factsheets/is_lantern.cfm Radioactivity in Lantern Mantles]. Australian Radiation Protection and Nuclear Safety Agency</ref> Production of gas mantles has led to some safety concerns during [[Gas mantle#Safety concerns|manufacture]].

The element has no known biological role. Humans typically consume three micrograms per day of thorium. Of this, 99.98% does not remain in the body. Out of the thorium that does remain in the body, three quarters of it accumulates in the [[skeleton]]. A number of thorium compounds are chemically moderately [[toxic]]. People who work with thorium compounds are at a risk of [[dermatitis]]. It can take as much as thirty years after the ingestion of thorium for symptoms to manifest themselves.<ref name="Nature's Building Blocks">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks|year=2011}}</ref>

== See also ==
* [[Accelerator-driven sub-critical reactor]]
* [[Actinides in the environment]]
* [[India's three stage nuclear power programme]]
* [[Liquid fluoride thorium reactor]]
* [[Materials science in science fiction#Thorium|Materials science in science fiction]]
* [[Sylvania Electric Products explosion]]

== References ==
{{Reflist|35em}}

== Bibliography ==
* <!-- Ha -->{{cite book
| title = The Chemistry of the Actinide and Transactinide Elements
| editor1-first = Lester R.
| editor1-last = Morss
| editor2-first = Norman M.
| editor2-last = Edelstein
| editor3-last = Fuger
| editor3-first = Jean
| last = Wickleder
| first = Mathias S.
| coauthors = Fourest, Blandine; Dorhourt, Peter K.
| chapter = Thorium
| publisher = [[Springer Science+Business Media]]
| year = 2006
| isbn = 1-4020-3555-1
| edition = 3rd
| ref = {{sfnref|Wickleder|2006}}
}}

==Further reading==
* <!-- Ha -->{{cite book
| title = Super Fuel: Thorium, the Green Energy Source for the Future
| first = Richard
| last = Martin
| pages = 240
| publisher = [[Palgrave MacMillan]]
| year = 2012
| isbn = 978-0-230-11647-4 (hardback)
| edition = 1st
}}
* <!-- Ha -->{{cite book
| title = Thorium: Energy Cheaper than Coal
| first = Robert
| last = Hargraves
| pages = 482
| publisher = [[CreateSpace Independent Publishing Platform]]
| date = 25 July 2012
| isbn = 978-1478161295
}}

== External links ==
{{Commons|Thorium}}
{{Wiktionary|thorium}}
* [http://www.itheo.org International Thorium Energy Organisation – IThEO.org]
* [http://www.euronuclear.org/info/encyclopedia/d/decaybasinnatural.htm European Nuclear Society – Natural Decay Chains]
* [http://www.atsdr.cdc.gov/tfacts147.html ATSDR CDC ToxFAQs: health questions about thorium]
* [http://www.world-nuclear.org/info/inf62.html FactSheet on Thorium], [[World Nuclear Association]]
* [http://www.thorium.tv/en/index.php Thorium TV – A review of the element]
* [http://energyfromthorium.com/ EnergyFromThorium.com – Content-rich site on Thorium as a future energy source, and its extraction technology]
* [http://www.ted.com/talks/kirk_sorensen_thorium_an_alternative_nuclear_fuel.html TED talk by former NASA engineer Kirk Sorensen about Thorium energy production (video)]
* [http://www.youtube.com/watch?v=Nl5DiTPw3dk India's experimental Thorium Fuel Cycle Nuclear Reactor (NDTV Report)]
* [http://www.youtube.com/watch?v=P9M__yYbsZ4 Thorium Remix 2011 – 120 minute Creative Commons Share-Alike documentary on Thorium as an energy source]
* [http://www.guardian.co.uk/environment/2011/nov/01/india-thorium-nuclear-plant Newspaper article about thorium power in India]
* [http://www.telegraph.co.uk/finance/comment/ambroseevans_pritchard/9784044/China-blazes-trail-for-clean-nuclear-power-from-thorium.html China Blazes Trail for Clean Nuclear Power from Thorium]
* [http://www.periodicvideos.com/videos/090.htm Thorium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://pubs.usgs.gov/circ/1336/ Thorium Deposits of the United States—Energy Resources for the Future?] ([[United States Geological Survey|USGS]], 2009)

{{Clear}}
{{Compact periodic table}}
{{Thorium compounds}}

[[Category:Chemical elements]]
[[Category:Actinides]]
[[Category:Nuclear materials]]
[[Category:Nuclear fuels]]
[[Category:Carcinogens]]
[[Category:Thorium]]

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