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History

The Reverend Has Morten Thrane 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 Jons Jakob Berzelius for examination in 1828.  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.  Esmark's mineral is now known as thorite (ThSiO₄).

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.

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.

Characteristics

When pure, thorium is a silvery white metal that retains its luster for several months.  However, when it is contaminated with the oxide, 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).  When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

Occurrence

Thorium makes up about 0.0007% of the earth's crust and is primarily obtained from thorite, thorianite (ThO₂) and monazite [(Ce, La, Th, Nd, Y)PO₄].   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.

Distribution

Present knowledge of the distribution of Thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.  Under the prevailing estimate, Austrailia 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):

• 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).

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

Extraction

Large deposits of thorium minerals have been reported in New England and elsewhere, but these have not yet been exploited. Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum.  Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO₂ along with rare-earth minerals, 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.

Applications

• As an alloying element in magnesium, 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 designs of nuclear reactor incorporate thorium in their nuclear 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.

Thorium Dioxide (ThO₂):

Thorium dioxide (ThO₂), one of thorium's compounds, has many uses.  It is primarily used in a type of lantern mantel known as a Welsbach mantle.   This mantle, which also contains about 1% cerium oxide, glows with a bright white light when it is heated in a gas flame.  Thorium dioxide has a very high melting point, about 3300°C, and is used to make high temperature crucibles. Thorium dioxide is also used to make glass with a high index of refraction that is used to make high quality camera lenses.

Thorium dioxide is used as a catalyst in the production of sulfuric acid (H₂SO₄), in the cracking of petroleum products and in the conversion of ammonia (NH₃) to nitric acid (HNO₃).

• 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.
• 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.

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 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 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 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.  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, 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 reactor are efforts in this direction.

Compounds

Thorium dioxide is a material for heat-resistant ceramics, e.g., for high temperature laboratory crucibles.  When added to glass, it helps increase refractive index and decrease dispersion.  Such glass finds application in high-quality lenses for cameras and scientific instruments.

Thorium dioxide (ThO₂) d thorium nitrate (Th(NO₃)₄) were used in mantles of portable gas lights, including natural gas lamps, oil lamps and camping lights.  These mantles glow with an intense white light (unrelated to radioactivity) when heated in a gas flame, and its color could be shifted to yellow by addition of cerium.

Thorium dioxide was used to control the grain size of tungsten metal used for spirals of electric lamps.  Thoriated tungsten elements were also found in the active ingredient of microwave frequencies and were applied in microwave ovens and radars.

Thorium dioxide has been used as a catalyst in the conversion of ammonia (NH₃) to nitric acid (HNO₃), in petroleum cracking and in producing sulfuric acid.  It is the active ingredient of Thorotrast, which was used as part of X-ray diagnostics.  This use has been abandoned due to the carcinogenic nature of Thortrast.

Despite its radioactivity, thorium fluoride (ThF₄) is used as an antireflection material in multilayered optical coatings.  It has excellent optical transparency and its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.  Thorium fluoride was also used in manufacturing carbon arc lamps, which provided high-intensity illumination for movie projectors and search lights.

Isotopes

Naturally occurring thorium is composed of one isotope: ²³²Th.  Thirty radioisotopes have been characterized, with the most abundant and/or stable being ²³²Th, an alpha emitter, 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.

The known isotopes of thorium range in atomic weight from 210 amu (²¹⁰Th) to 236 amu (²³⁶Th).

²³²Th goes through six alpha and four beta decay steps before becoming the stable isotope ²⁰⁸Pb. ²³²Th is sufficiently radioactive to expose a photographic plate in a few hours. Thorium disintegrates with the production of "thoron" (²²⁰Rn), which is an alpha emitter and presents a radiation hazard.  Good ventilation of areas where thorium is stored or handled is therefore essential.

When bombarded with neutrons, ²³²Th becomes ²³³Th, which eventually decays into ²³³U through a series of beta decays.   ²³³U is a fissionable material and can be used as a nuclear fuel.

Thorium's most stable isotope, thorium-232, decays into radium-228 through alpha decay or decays through spontaneous fission.

Precautions

Powdered thorium metal is often pyrophoric and should be handled carefully.  When heated in air, thorium turnings ignite and burn brilliantly with a white light.

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric.

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.

Popular Culture

David Hahn, the so-called "radioactive boy scout," bombarded thorium from lantern mantles with neutrons to produce small quantities of fissionable material in his backyard. He had to abandon his project when he began to detect elevated radiation levels several houses away from his own.