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Thorium
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Thorium is a chemical element with 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.
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).
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Encyclopedia
Thorium is a chemical element with 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.
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). 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 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:
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 in gas tungsten arc welding electrodes.
- Used to control the grain size of tungsten used for electric lamps.
- Used in heat-resistant ceramics like 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:
- 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.
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.
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.
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 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, the most common being the rare-earth thorium-phosphate mineral monazite, which may contain up to about 12% thorium oxide. Thorium-containing monazite(Ce) occurs in Africa, Antarctica, Australia, Europe, India, North America, and South America.
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.
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. 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 approx 300,000 metric tonnes each; i.e. each country possessing 25% of the world's thorium reserves. 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 world’s reserve of monazite is estimated to be in the range of 12 million tonnes of
which nearly 8 million tonnes occur with the heavy minerals in the beach sands of India in the
States of Kerala, Tamil Nadu, Andhra Pradesh and Orissa.
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:
In the RAR category, the deposits in Brazil, Turkey and India are in the range of 0.60, 0.38
and 0.32 million tonnes respectively. The thorium deposits in India has recently been reported
to be in the range 0.65 million tonnes.
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):
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):
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 233U, which is fissile. Hence, like 238U, it is fertile. Theoretically thorium is more suitable fuel source than uranium: thorium is at least 4-5 times more abundant in nature than all of uranium isotopes combined; thorium is fairly evenly spread around Earth with a lot of countries having huge supplies of it; preparation of thorium fuel does not require difficult & expensive enrichment process; thorium fuel cycle creates mainly Uranium-233 contaminated with Uranium-232 which makes it ill suited to weapons proliferation; elimination of the nuclear waste problem in established Molten Salt Reactor (MSR) designs.
Thorium can and has been used both in modified traditional Generation III reactor designs and in 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.
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.
Isotopes
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 7.6 eV.
The known isotopes of thorium range in atomic weight from 210 u (210Th) to 236 u (236Th).
Precautions
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.
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 ion such as nitrate, chloride, hydroxide, or carbonate, returning the thorium to an aqueous phase. Finally, the thorium is precipitated and collected.
See also
Footnotes
External links
- , a commercial product which claimed to destroy odours 'forever.' Made with thorium-232.
- Blog, discussion forum and document repository
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