thorium

 

(thôr'ēəm) [from Thor], radioactive chemical element; symbol Th; at. no. 90; at. wt. 232.0381; m.p. about 1,750°C; b.p. about 4,790°C; sp. gr. 11.7 at 20°C; valence +4.

Thorium is a soft, ductile, lustrous, silver-white, radioactive metal. At ordinary temperatures it has a face-centered cubic crystalline structure. It is a member of the actinide series in Group 3 of the periodic table and is sometimes classed as one of the rare-earth metals. When pure, the metal is stable and resists oxidation, but it is usually contaminated with small amounts of the oxide, which cause it to tarnish rapidly. It reacts slowly with water and is attacked only by hydrochloric acid among the common acids. The finely divided metal readily ignites when heated, burning with a brilliant white flame; the oxide formed has the highest melting point of all oxides. Thorium forms numerous compounds with other elements.

Thorium is widely distributed in small amounts in the earth's crust, being about half as abundant as lead and three times as abundant as uranium. The chief commercial source of thorium is monazite sands obtained from India and Brazil. It is also found in the minerals thorite (thorium silicate, ThSiO₄) and thorianite (mixed thorium and uranium oxides). Vast deposits of low-grade thorium ore in New Hampshire are a potential source. Thorium metal is isolated with difficulty; it is obtained from certain of its compounds by electrolysis or by chemical reduction. Thorium is used in magnesium alloys and in tungsten filaments for light bulbs and electronic tubes. The most important thorium compound is the oxide (thoria, ThO₂), which is the major incandescent component of the Welsbach mantle; it is also used in crucibles, in special highly refractive optical glass, and in catalysts for several industrially important chemical reactions. Important uses of the element result from its natural radioactivity.

There are 26 known radioactive isotopes, only 12 of which have half-lives greater than 1 sec. The most stable is thorium-232 (half-life 1.41 × 10¹⁰ years); it is the major component of naturally occurring thorium, which has atomic weight 232.038 atomic mass units. Thorium-232 undergoes natural disintegration and eventually is converted through a 10-step chain of isotopes to lead-208, a stable isotope; alpha and beta particles are emitted during this decay. One intermediate product is the gas radon-220, also called thorium emanation or thoron. Thorium and its decay products are sometimes used in radiotherapy. Although thorium-232 is not itself a nuclear reactor fuel since it will not sustain a chain reaction, it is expected to become increasingly important for conversion into the fissionable fuel uranium-233.

Thorium-232 can react with a thermal (slow) neutron to form thorium-233, emitting a gamma ray. Thorium-233 decays (half-life about 22 min) to protactinium-233, emitting a beta particle. The protactinium-233 decays (half-life about 27 days) with another beta particle emission to uranium-233. Fission of the uranium-233 can provide neutrons to start the cycle again. This cycle of reactions is known as the thorium cycle. Nuclear reactors that use a cycle like this to produce fuel are called breeder reactors. Thorium was discovered in 1828 by Jöns Jakob Berzelius but had few uses until the invention of the Welsbach mantle in 1885.

90 actinium ← thorium → protactinium Ce ↑ Th ↓ (Uqn) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	thorium, Th, 90
Chemical series	Actinides
Group, Period, Block	n/a, 7, f
Appearance	silvery white
Standard atomic weight	232.03806(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
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	cubic face centered
Oxidation states	4 (weakly basic oxide)
Electronegativity	1.3 (scale Pauling)
Ionization energies (more)	1st:  587  kJ·mol−1
	2nd:  1110  kJ·mol−1
	3rd:  1930  kJ·mol−1
Atomic radius	180  pm
Miscellaneous
Magnetic ordering	no data
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
Selected 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 (IPA: /ˈθɔːriəm/) is a chemical element in the periodic table that has the symbol Th and atomic number 90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium.

Notable characteristics

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)^[1]. 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 (2946 K) between the melting point and boiling point.

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

Applications

Applications of thorium:

• As an alloying element in magnesium, used in aircraft engines, imparting high strength and creep resistance at elevated temperatures.
• Thorium is used to coat tungsten wire used in electronic equipment, improving the electron emission of heated cathodes.
• Thorium has been used in gas tungsten arc welding electrodes and heat-resistant ceramics.
• Uranium-thorium age dating has been used to date hominid fossils.
• As a fertile material for producing nuclear fuel. In particular, the proposed energy amplifier reactor design would employ thorium. Since thorium is more abundant than uranium, some nuclear reactor designs 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 lead or depleted uranium.
• Thorium may be used in nuclear reactors instead of uranium as fuel. This produces less transuranic waste.

Applications of thorium dioxide (ThO₂):

• Mantles in portable gas lights. These mantles glow with a dazzling light (unrelated to radioactivity) when heated in a gas flame.
• Used to control the grain size of tungsten used for electric lamps.
• Used for high-temperature laboratory crucibles.
• Added to glass, it helps create glasses of a high refractive index and with low dispersion. Consequently, they find application in high-quality lenses for cameras and scientific instruments.
• Has been used as a catalyst:
  • In the conversion of ammonia to nitric acid.
  • In petroleum cracking.
  • In producing sulfuric acid.
• Thorium dioxide is the active ingredient of Thorotrast, which was used as part of X-ray diagnostics. This use has been abandoned due to the carcinogenic nature of Thorotrast.

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.^[2] 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 mantle in 1885.

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. ^[3]

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

Occurrence

Thorium is found in small amounts in most rocks and soils, 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 contains up to about 12% thorium oxide. There are substantial deposits in several countries. ²³²Th decays very slowly (its half-life is about three times the age of the earth) 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. India is believed to have 25% of the world's Thorium reserves. ^[4]

See also thorium minerals.

Distribution

Present knowledge of the distribution of Thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.^[5] Under the prevailing estimate, Australia and India have particularly large reserves of thorium.

• The prevailing estimate of the economically available thorium reserves comes from the US Geological Survey, Mineral Commodity Summaries (1997-2006):^[6][7]

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

• 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)^[8]

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

The two sources vary wildly for countries such as Brazil, Turkey, and Australia.

Thorium as a nuclear fuel

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 uranium-233 (²³³U), which is fissile. Hence, like ²³⁸U, it is fertile. In one significant respect ²³³U is better than the other two fissile isotopes used for nuclear fuel, ²³⁵U and plutonium-239 (²³⁹Pu), because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (²³⁵U or ²³⁹Pu), a breeding cycle similar to, but more efficient than that currently possible with the ²³⁸U-to-²³⁹Pu cycle (in slow-neutron reactors), can be set up. The ²³²Th absorbs a neutron to become ²³³Th which normally decays to protactinium-233 (²³³Pa) and then ²³³U. The irradiated fuel can then be unloaded from the reactor, the ²³³U separated from the thorium (a relatively simple process since it involves chemical instead of isotopic separation), and fed back into another reactor as part of a closed nuclear fuel cycle.

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.

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.

India, having about 25% of the world's reserves ^[4], has planned its nuclear power program to eventually use thorium exclusively, phasing out uranium as a feed stock. This ambitious plan uses both fast and thermal breeder reactors. The Advanced Heavy Water Reactor and KAMINI reactor are efforts in this direction.

In 2007, Norway was debating whether or not to focus on Thorium plants.

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

Isotopes

Main article: isotopes of thorium

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-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 3.5 eV. ^[9]

The known isotopes of thorium range in atomic weight from 210 u (²¹⁰Th)^[10] to 236 u (²³⁶Th).

Precautions

Powdered thorium metal is often 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 aerosolized 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.

Thorium Extraction

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 anion such as nitrate, chloride, hydroxide, or carbonate, returning the thorium to an aqueous phase. Finally, the thorium is precipitated and collected.^[11]

See also

• David Hahn, who produced small quantities of fissionable material in his backyard.
• Periodic table
• Nuclear reactor
• Decay chain
• Sylvania Electric Products explosion
• Thorium's entries at fictional applications of real materials.

References

1. ^ Emsley, John (2001). Nature's Building Blocks, (Hardcover, First Edition), Oxford University Press, page 441. ISBN 0198503407. 
2. ^ Thorium. BBC.co. Retrieved on 2007-01-18.
3. ^ 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. Retrieved on 2006-05-06. 
4. ^ ^{a b} US approves Indian nuclear deal. BBC News (2006-12-09).
5. ^ K.M.V. Jayaram. An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future.
6. ^ U.S. Geological Survey, Mineral Commodity Summaries - Thorium.
7. ^ Information and Issue Briefs - Thorium. World Nuclear Association. Retrieved on 2006-11-01.
8. ^ IAEA: Thorium fuel cycle -- Potential benefits and challenges, pp 45(table 8), 97(ref 78). 
9. ^ Phys. Rev. C 73 044326 (April 2006)
10. ^ Phys. Rev. C 52, 113–116 (1995)
11. ^ Crouse, David; Brown, Keith (December 1959). "The Amex Process for Extracting Thorium Ores with Alkyl Amines".Industrial & Engineering Chemistry 51 (12): 1461. Retrieved on 2007-03-09

• Los Alamos National Laboratory — Thorium
• WebElements.com — Thorium
• The Uranium Information Centre provided some of the original material in this article.
• European Nuclear Society — Natural Decay Chains

External links

• Thorium information page
• New Age Nuclear: article on thorium reactors | Cosmos Magazine
• ATSDR ToxFAQs — Thorium
• USGS data — Thorium
• The Endless Refrigerator/Freezer Deodorizer, a commercial product which claimed to destroy odours 'forever.' Made with thorium-232.
• Is thorium the answer to our energy crisis?

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