Thorium

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Thorium

Thorium 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 lustre 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 one of the highest melting points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

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

Applications

Applications of thorium:

Mantles in portable gas lights. These mantles glow with a dazzling light (unrelated to radioactivity) when heated in a gas flame.
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 have lead or depleted uranium.
Thorium may be used in subcritical reactors instead of uranium as fuel. This produces less waste and cannot meltdown.

Applications of thorium dioxide (ThO₂):

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

Thorium was discovered in 1828 by the Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the lantern 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.

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 6 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.

See also Thorium minerals.

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 the need for fast neutron reactors, holds considerable potential long-term. Thorium is significantly more abundant than uranium, so it is a key factor in the sustainability of nuclear energy.

India has particularly large reserves of thorium, and so have planned their nuclear power program to eventually use it exclusively, phasing out uranium as an input material. This ambitious plan uses both fast and thermal breeder reactors. The Advanced Heavy Water Reactor and KAMINI reactor are efforts in this direction.

The current thorium mineral reserve estimates (in tons)^*:

360,000 India^*
300,000 Australia
170,000 Norway
160,000 United States
100,000 Canada
35,000 South Africa
16,000 Brazil
95,000 Others

Isotopes

Naturally occurring thorium is composed of one isotope: ²³²Th. Twenty five 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-lifes that are less than thirty days and the majority of these have half lifes that are less than ten minutes. This element also has one meta state.

The isotopes of thorium range in atomic weight from 212 amu (²¹²Th) to 236 amu (²³⁶Th).

Precautions

Powdered thorium metal is often pyrophoric and should be handled carefully. The thorium decay chain ends with an isotope of lead (²⁰⁸Pb), but passes through an isotope of radon (²²⁰Rn) (also called "thoron")^*. Radon gas is a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential.

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.

In popular culture

See also thorium's entries at fictional applications of real materials.

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.

In 1999, a group of University of Chicago students taking part in the annual scavenger hunt built a small, working nuclear reactor.

References

Los Alamos National Laboratory — Thorium
European Nuclear Society — Natural Decay Chains
van Arkel, A.E., and de Boer, J.H., 1925, Preparation of pure titanium, zirconium, hafnium, and thorium metal: Zeitschrift für Anorganische und Allgemeine Chemie, v. 148, p. 345–350.

External links

Energy from Thorium blog
Thorium information page
Thorium Power, Inc. — Developer of proliferation-resistant thorium fuels
WebElements.com — Thorium
ATSDR ToxFAQs — Thorium
USGS data — Thorium
The Uranium Information Centre provided some of the original material in this article.
The Endless Refrigerator/Freezer Deodorizer, a commercial product which claims to destroy odours 'forever.' Made with Th-232.

See: Periodic table, nuclear reactor, Decay chain

Chemical elements | Actinides | Nuclear materials

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