Thorium

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90 actinium ← thorium → protactinium
Ce
↑
Th
↓
(Uqn)
General
Name, Symbol, Number thorium, Th, 90
Element category Actinides
Group, Period, Block n/a, 7, f
Appearance silvery white
Standard atomic weight 232.0381(2)  g·mol−1
Electron configuration [Rn] 6d2 7s2
Electrons per shell 2, 8, 18, 32, 18, 10, 2
Physical properties
Phase solid
Density (near r.t.) 11.7  g·cm−3
Melting point 2115 K
(1842 °C, 3348 °F)
Boiling point 5061 K
(4788 °C, 8650 °F)
Heat of fusion 13.81  kJ·mol−1
Heat of vaporization 514  kJ·mol−1
Specific heat capacity (25 °C) 26.230  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2633 2907 3248 3683 4259 5055
Atomic properties
Crystal structure face centered cubic
Oxidation states 4, 3, 2
(weakly basic oxide)
Electronegativity 1.3 (Pauling scale)
Ionization energies
(more)		 1st:  587  kJ·mol−1
2nd:  1110  kJ·mol−1
3rd:  1930  kJ·mol−1
Atomic radius 179  pm
Covalent radius 206±6  pm
Miscellaneous
Magnetic ordering paramagnetic[1]
Electrical resistivity (0 °C) 147 nΩ·m
Thermal conductivity (300 K) 54.0  W·m−1·K−1
Thermal expansion (25 °C) 11.0  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 2490 m/s
Young's modulus 79  GPa
Shear modulus 31  GPa
Bulk modulus 54  GPa
Poisson ratio 0.27
Mohs hardness 3.0
Vickers hardness 350  MPa
Brinell hardness 400  MPa
CAS registry number 7440-29-1
Most-stable isotopes
Main article: Isotopes of thorium
iso NA half-life DM DE (MeV) DP
228Th syn 1.9116 years α 5.520 224Ra
229Th syn 7340 years α 5.168 225Ra
230Th syn 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
References

Thorium (pronounced /ˈθɔəriəm/) is a chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal, which has been successfully used as an alternative nuclear fuel to uranium in the molten-salt reactor experiment (MSR) for several years to produce thermal energy. Thorium is abundant on Earth and this type of reactor can be built to operate significantly cleaner than uranium based power plants as the waste products are much easier to handle. The benefits of the MSR are exactly the same (fuel abundancy and clean operation) as what nuclear fusion promises.

Contents

[edit] Characteristics

[edit] Physical

When pure, thorium is a silvery-white metal which is air-stable and retains its luster for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. The purest specimens often contain several tenths of a percent of the oxide. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400 °C from a face-centered cubic to a body-centered cubic structure. Powdered thorium metal is often pyrophoric and should be carefully handled. When heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Thorium has the largest liquid range of any element: 2946 °C between the melting point and boiling point. [2]

[edit] Chemical

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric.[2] It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride ion.[3]

[edit] Compounds

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

Thorium dioxide has the highest melting point (3300 °C) of all oxides.[5]

Thorium(IV) nitrate and thorium(IV) fluoride are known in their hydrated forms: Th(NO3)4.4H2O and ThF4.4H2O, respectively. The thorium center has square planar geometry.[4] Thorium(IV) carbonate, Th(CO3)2, is also known.[4]

When treated with potassium fluoride and hydrofluoric acid, Th4+ forms the complex anion ThF62−, which precipitates as an insoluble salt, K2ThF6.[3]

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.[3]

In the presence of phosphate anions, Th4+ forms precipitates of various compositions, which are insoluble in water and acid solutions.[3]

[edit] Isotopes

Main article: isotopes of thorium

Naturally occurring thorium is composed mainly of one isotope: 232Th. 230Th occurs as the daughter product of 238U decay. Twenty-seven radioisotopes have been characterized, with the most abundant and/or stable being 232Th with a half-life of 14.05 billion years, 230Th with a half-life of 75,380 years, 229Th with a half-life of 7340 years, and 228Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy of 7.6 eV.[6]

The known isotopes of thorium range in atomic weight from 210 u (210Th) to 236 u (236Th).[7]

[edit] Applications

Applications of thorium:[2]

Applications of thorium dioxide (ThO2):

[edit] Thorium as a nuclear fuel

Main article: Thorium fuel cycle

Thorium, as well as uranium and plutonium, can be used as fuel in a nuclear reactor. Although not fissile itself, 232Th will absorb slow neutrons to produce 233U, which is fissile. Hence, like 238U, it is fertile. Theoretically thorium is a more suitable fuel source than uranium. It is at least 4-5 times more abundant in nature than all of uranium isotopes combined and is fairly evenly spread around Earth, with many countries having large supplies of it. Also, preparation of thorium fuel does not require difficult and expensive enrichment process. The thorium fuel cycle creates mainly Uranium-233 which can be used for making nuclear weapons, and since there are no neutrons from spontaneous fission of U-233, U-233 can be used easily in a gun-type nuclear bomb[10] Thorium can and has been used to power nuclear energy plants using both the modified traditional Generation III reactor design and prototype Generation IV reactor designs.

When using 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 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, and the effort required seems unlikely while (or where) abundant uranium is available.

Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without fast neutron reactors, holds considerable potential long-term benefits. Thorium is significantly more abundant than uranium, and is a key factor in sustainable nuclear energy. Perhaps more importantly, thorium produces several orders of magnitude less long-lived radioactive waste.

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

India's Kakrapar-1 reactor is the world's first reactor which utilizes thorium rather than depleted uranium to achieve power flattening across the reactor core.[11] India, which has about 25% of the world's thorium reserves, is developing a 300 MW prototype of a thorium-based Advanced Heavy Water Reactor (AHWR). The prototype is expected to be fully operational by 2011, following which five more reactors will be constructed.[12] India currently envisages to meet 30% of its electricity demand through thorium-based reactors by 2030.[13]

In 2007, Norway was 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 Telemark county.

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

[edit] History

M. T. Esmark found a black mineral on Løvøy Island, Norway and gave a sample to Professor Jens Esmark, a noted mineralogist who was not able to identify it, so he sent a sample to the Swedish chemist Jöns Jakob Berzelius for examination in 1828.[14] Berzelius analyzed it and named it after Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the gas mantle in 1885.

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.[15]

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.[16]

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.

[edit] Occurrence

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

Thorium is found in small amounts in most rocks and soils, where it is about four times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 12 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO4), thorianite (ThO2 + UO2) and monazite. The latter is most common and may contain up to about 12% thorium oxide. Thorium-containing monazite(Ce) occurs in Africa, Antarctica, Australia, Europe, India, North America, and South America.[17][2]

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.

[edit] 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.[18]

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.[2]

[edit] Distribution

Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.[19] There are two sets of estimates that define world thorium reserves, one set by the US Geological Survey (USGS) and the other supported by reports from the OECD and the International Atomic Energy Agency (the IAEA). Under the USGS estimate, Australia and India have particularly large reserves of thorium. India and Australia are believed to possess about 300,000 metric tonnes each; i.e. each country possessing 25% of the world's thorium reserves.[20] 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 IAEA also states that recent reports have upgraded India's thorium deposits up from approximately 300,000 metric tonnes to 650,000 metric tonnes.[21] Therefore, the IAEA and OECD appear to conclude that Brazil and India may actually possess the lion's share of world's thorium deposits.

  • The prevailing estimate of the economically available thorium reserves comes from the US Geological Survey, Mineral Commodity Summaries (1997-2006):[22][23]
Country Th Reserves (tonnes)     Th Reserve Base (tonnes)
Australia 300,000           340,000          
India 290,000 300,000
Norway 170,000 180,000
United States 160,000 300,000
Canada 100,000 100,000
South Africa 35,000 39,000
Brazil 16,000 18,000
Malaysia 4,500 4,500
Other Countries 95,000 100,000
World Total 1,200,000 1,400,000

Note: The 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):[24]
Country RAR Th (tonnes)   EAR Th (tonnes)
Brazil 606,000           700,000          
Turkey 380,000 500,000
India 319,000     —
United States 137,000 295,000
Norway 132,000 132,000
Greenland 54,000 32,000
Canada 45,000 128,000
Australia 19,000     —
South Africa 18,000     —
Egypt 15,000 309,000
Other Countries   505,000     —
World Total 2,230,000 2,130,000

[edit] Precautions

See Actinides in the environment for details of the environmental aspects of thorium.

Powdered thorium metal will often ignite spontaneously in air (it is pyrophoric) and should be handled carefully. Natural thorium decays very slowly compared to many other radioactive materials, and the alpha radiation emitted 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 an aerosol of thorium can lead to increased risk of cancers of the lung, pancreas and blood. Exposure to thorium internally leads to increased risk of liver diseases. This element has no known biological role. See also Thorotrast.

[edit] See also

[edit] References

  1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81th edition, CRC press.
  2. ^ a b c d e C. R. Hammond (2004). The Elements, in Handbook of Chemistry and Physics 81th edition. CRC press. ISBN 0849304857. 
  3. ^ a b c d Earl K. Hyde (1960). The radiochemistry of thorium. Subcommittee on Radiochemistry, National Academy of Sciences—National Research Council. http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf. 
  4. ^ a b c "Toxicological Profile Information Sheet". Department of Health and Human Services. http://www.atsdr.cdc.gov/toxprofiles/tp147-c3.pdf. Retrieved on 2009-05-21. 
  5. ^ Emsley, John (2001). Nature's Building Blocks ((Hardcover, First Edition) ed.). Oxford University Press. pp. 441. ISBN 0198503407. 
  6. ^ B. R. Beck et al. (2007). "Energy Splitting of the Ground-State Doublet in the Nucleus 229Th". Phys. Rev. Lett. 98: 142501. doi:10.1103/PhysRevLett.98.142501. 
  7. ^ J. Uusitalo et al. (1995). "α decay of the new isotopes 210Th and 211Th". Phys. Rev. C 52: 113. doi:10.1103/PhysRevC.52.113. 
  8. ^ ed. by Michael M. Avedesian ... Prepared under the direction of the ASM International Handbook Committee. (1999). "Microstructure of Magnesium and Magnesium Alloys". Magnesium and magnesium alloys. Materials Park, OH: ASM International. p. 28. ISBN 9780871706577. http://books.google.de/books?id=0wFMfJg57YMC&pg=PA28. 
  9. ^ Larry Jeffus. (2003). "Types of Tungsten". Welding : principles and applications. Clifton Park, N.Y.: Thomson/Delmar Learning. p. 350. ISBN 9781401810467. http://books.google.de/books?id=zeRiW7en7HAC&pg=RA1-PA750. 
  10. ^ R. Wilson (1998). "Accelerator Driven Subcritical Assemblies". Report to Energy Environment and Economy Committee, U.S. Global Strategy Council. http://phys4.harvard.edu/~wilson/publications/ppaper703.html. 
  11. ^ Thorium: Cleaner Nuclear Power?
  12. ^ Development work on 300 MW advanced heavy water reactor at advanced stage
  13. ^ Indian Thorium based reactor design complete
  14. ^ "Thorium". BBC.co. http://www.bbc.co.uk/dna/h2g2/A3768861. Retrieved on 2007-01-18. 
  15. ^ Simmons, John Galbraith (1996). The Scientific 100. Seacaucus NJ: Carol. p. 19. 
  16. ^ van Arkel, A.E.; de Boer, J.H. (1925). "Preparation of pure titanium, zirconium, hafnium, and thorium metal". Zeitschrift für Anorganische und Allgemeine Chemie 148: 345–350. 
  17. ^ "Monazite-(Ce): Monazite-(Ce) mineral information and data". http://www.mindat.org/min-2751.html. Retrieved on 18 May 2009. 
  18. ^ Crouse, David (1959). "The Amex Process for Extracting Thorium Ores with Alkyl Amines". Industrial & Engineering Chemistry 51: 1461. doi:10.1021/ie50600a030. 
  19. ^ K.M.V. Jayaram. "An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future". http://www.iaea.org/inis/aws/fnss/fulltext/0412_1.pdf. 
  20. ^ "US approves Indian nuclear deal". BBC News. 2006-12-09. http://news.bbc.co.uk/2/hi/south_asia/6219998.stm. 
  21. ^ IAEA: Thorium fuel cycle — Potential benefits and challenges. pp. 45. http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf. 
  22. ^ "U.S. Geological Survey, Mineral Commodity Summaries - Thorium". http://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs. 
  23. ^ "Information and Issue Briefs - Thorium". World Nuclear Association. http://www.world-nuclear.org/info/inf62.htm. Retrieved on 2006-11-01. 
  24. ^ IAEA: Thorium fuel cycle — Potential benefits and challenges. pp. 45(table 8), 97(ref 78). http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf. 

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