Thorium dioxide

Last updated
Thorium dioxide
IUPAC names
Thorium dioxide
Thorium(IV) oxide
Other names
Thorium anhydride
3D model (JSmol)
ECHA InfoCard 100.013.842 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
Molar mass 264.037 g/mol [1]
Appearancewhite solid [1]
Odor odorless
Density 10.0 g/cm3 [1]
Melting point 3,350 °C (6,060 °F; 3,620 K) [1]
Boiling point 4,400 °C (7,950 °F; 4,670 K) [1]
insoluble [1]
Solubility insoluble in alkali
slightly soluble in acid [1]
16.0·10−6 cm3/mol [2]
2.200 (thorianite) [3]
Fluorite (cubic), cF12
Fm3m, No. 225
a = 559.74(6) pm [4]
Tetrahedral (O2−); cubic (ThIV)
65.2(2) JK1mol1
1226(4) kJ/mol
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard RA: Radioactive. E.g. plutoniumThorium dioxide
Radiation warning symbol 3.svg
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
400 mg/kg
Related compounds
Other cations
Hafnium(IV) oxide
Cerium(IV) oxide
Related compounds
Protactinium(IV) oxide
Uranium(IV) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)
Infobox references

Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in color. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. [4] Thorianite is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides. Only a few elements (including tungsten and carbon) and a few compounds (including tantalum carbide) have higher melting points. [5] All thorium compounds are radioactive because there are no stable isotopes of thorium.


Structure and reactions

Thoria exists as two polymorphs. One has a fluorite crystal structure. This is uncommon among binary dioxides. Other binary oxides with fluorite structure include cerium dioxide, uranium dioxide and plutonium dioxide).[ clarification needed ] The band gap of thoria is about 6  eV. A tetragonal form of thoria is also known.

Thorium dioxide is more stable than thorium monoxide (ThO). [6] Only with careful control of reaction conditions can oxidation of thorium metal give the monoxide rather than the dioxide. At extremely high temperatures, the dioxide can convert to the monoxide either by a disproportionation reaction (equilibrium with liquid thorium metal) above 1,850 K (1,580 °C; 2,870 °F) or by simple dissociation (evolution of oxygen) above 2,500 K (2,230 °C; 4,040 °F). [7]


Nuclear fuels

Thorium dioxide (thoria) can be used in nuclear reactors as ceramic fuel pellets, typically contained in nuclear fuel rods clad with zirconium alloys. Thorium is not fissile (but is "fertile", breeding fissile uranium-233 under neutron bombardment); hence, it must be used as a nuclear reactor fuel in conjunction with fissile isotopes of either uranium or plutonium. This can be achieved by blending thorium with uranium or plutonium, or using it in its pure form in conjunction with separate fuel rods containing uranium or plutonium. Thorium dioxide offers advantages over conventional uranium dioxide fuel pellets, because of its higher thermal conductivity (lower operating temperature), considerably higher melting point, and chemical stability (does not oxidize in the presence of water/oxygen, unlike uranium dioxide).

Thorium dioxide can be turned into a nuclear fuel by breeding it into uranium-233 (see below and refer to the article on thorium for more information on this). The high thermal stability of thorium dioxide allows applications in flame spraying and high-temperature ceramics.


Thorium dioxide is used as a stabilizer in tungsten electrodes in TIG welding, electron tubes, and aircraft gas turbine engines. As an alloy, thoriated tungsten metal is not easily deformed because the high-fusion material thoria augments the high-temperature mechanical properties, and thorium helps stimulate the emission of electrons (thermions). It is the most popular oxide additive because of its low cost, but is being phased out in favor of non-radioactive elements such as cerium, lanthanum and zirconium.

Thoria dispersed nickel finds its applications in various high temperature operations like combustion engines because it is a good creep resistant material. It can also be used for hydrogen trapping. [8] [9] [10] [11] [12]


Thorium dioxide has almost no value as a commercial catalyst, but such applications have been well investigated. It is a catalyst in the Ruzicka large ring synthesis. Other applications that have been explored include petroleum cracking, conversion of ammonia to nitric acid and preparation of sulfuric acid. [13]

Radiocontrast agents

Thorium dioxide was the primary ingredient in Thorotrast, a once-common radiocontrast agent used for cerebral angiography, however, it causes a rare form of cancer (hepatic angiosarcoma) many years after administration. [14] This use was replaced with injectable iodine or ingestable barium sulfate suspension as standard X-ray contrast agents.

Lamp mantles

Another major use in the past was in gas mantle of lanterns developed by Carl Auer von Welsbach in 1890, which are composed of 99 percent ThO2 and 1% cerium(IV) oxide. Even as late as the 1980s it was estimated that about half of all ThO2 produced (several hundred tonnes per year) was used for this purpose. [15] Some mantles still use thorium, but yttrium oxide (or sometimes zirconium oxide) is used increasingly as a replacement.

Glass manufacture

Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right) Yellowing of thorium lenses.jpg
Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)

When added to glass, thorium dioxide helps increase its refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments. [16] The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal. [17] Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides such as lanthanum oxide in almost all modern high-index glasses, as they provide similar effects and are not radioactive. [18]

Related Research Articles

The actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinoid series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinoid chemistry to refer to any actinoid.

Thorium Chemical element with atomic number 90

Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

Uranium Chemical element with atomic number 92

Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all isotopes of uranium are unstable; the half-lives of its naturally occurring isotopes range between 159,200 years and 4.5 billion years. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead, and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.

Nuclear fuel cycle Process of manufacturing and consuming nuclear fuel

The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

Breeder reactor type of fast neutron reactor that produces more fissile material than it consumes

A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile material, such as uranium-238 or thorium-232 that is loaded into the reactor along with fissile fuel. Breeders were at first found attractive because they made more complete use of uranium fuel than light water reactors, but interest declined after the 1960s as more uranium reserves were found, and new methods of uranium enrichment reduced fuel costs.

Molten salt reactor Type of nuclear reactor cooled by molten material

A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. A key characteristic of MSRs is their operation at or close to atmospheric pressure, rather than the 75-150 times atmospheric pressure of typical light-water reactors (LWR), hence reducing the large, expensive containment structures used for LWRs and eliminating a source of explosion risk. Another key characteristic of MSRs is higher operating temperatures than a traditional LWR, providing higher electricity-generation efficiency and, in some cases, process-heat opportunities. Relevant design challenges include the corrosivity of hot salts and the changing chemical composition of the salt as it is transmuted by reactor radiation.

Nuclear fuel material that can be used in nuclear fission or fusion to derive nuclear energy

Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.

Hot cathode Type of electrode.

In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. The heating element is usually an electrical filament heated by a separate electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment, and do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.

Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.

Uranium dioxide chemical compound

Uranium dioxide or uranium(IV) oxide (UO2), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used as MOX fuel. Prior to 1960, it was used as yellow and black color in ceramic glazes and glass.

Thorium fuel cycle nuclear fuel cycle using 232Th as fertile material, which absorbs neutrons to become into 233U (the nuclear fuel), which fissions to produce energy

The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, 232
, as the fertile material. In the reactor, 232
is transmuted into the fissile artificial uranium isotope 233
which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material, which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, 232
absorbs neutrons to produce 233
. This parallels the process in uranium breeder reactors whereby fertile 238
absorbs neutrons to form fissile 239
. Depending on the design of the reactor and fuel cycle, the generated 233
either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

Spent nuclear fuel nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant)

Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and depending on its point along the nuclear fuel cycle, it may have considerably different isotopic constituents.

Actinides in the environment refer to the sources, environmental behaviour and effects of actinides in Earth's environment. Environmental radioactivity is not limited solely to actinides; non-actinides such as radon and radium are of note. While all actinides are radioactive, there are a lot of actinides or actinide-relating minerals in the Earth's crust such as uranium and thorium. These minerals are helpful in many ways, such as carbon-dating, most detectors, X-rays, and more.

Thorium(IV) chloride chemical compound

Thorium(IV) chloride (ThCl4) is an inorganic chemical compound. In addition to the anhydrous ThCl4, two hydrates have been reported: ThCl4(H2O)4 and ThCl4(H2O)8. These hygroscopic salts are water-soluble and white, at room temperature. Similar to other thorium complexes thorium(IV) chloride has a high melting point 770 °C (1,418 °F) and a boiling point of 921 °C (1,690 °F). Like all the other actinides, thorium is radioactive and has sometimes been used in the production of nuclear energy. Thorium(IV) chloride does not appear naturally but instead is derived from Thorite, Thorianite, or Monazite which are naturally occurring formations.

Plutonium Chemical element with atomic number 94

Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon, and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous.

Liquid fluoride thorium reactor Type of nuclear reactor that uses molten material as fuel

The liquid fluoride thorium reactor is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based, molten, liquid salt for fuel. In a typical design, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat to a steam turbine or closed-cycle gas turbine.

Delay composition, also called delay charge or delay train, is a pyrotechnic composition, a sort of pyrotechnic initiator, a mixture of oxidizer and fuel that burns in a slow, constant rate that should not be significantly dependent on temperature and pressure. Delay compositions are used to introduce a delay into the firing train, e.g. to properly sequence firing of fireworks, to delay firing of ejection charges in e.g. model rockets, or to introduce a few seconds of time between triggering a hand grenade and its explosion. Typical delay times range between several milliseconds and several seconds.

Actinide chemistry branch of nuclear chemistry

Actinide chemistry is one of the main branches of nuclear chemistry that investigates the processes and molecular systems of the actinides. The actinides derive their name from the group 3 element actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. The actinide series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.

The Ames Project was a research and development project that was part of the larger Manhattan Project to build the first atomic bombs during World War II. It was founded by Frank Spedding from Iowa State College in Ames, Iowa as an offshoot of the Metallurgical Laboratory at the University of Chicago devoted to chemistry and metallurgy, but became a separate project in its own right. The Ames Project developed the Ames Process, a method for preparing pure uranium metal that the Manhattan Project needed for its atomic bombs and nuclear reactors. Between 1942 and 1945, it produced over 1,000 short tons (910 t) of uranium metal. It also developed methods of preparing and casting thorium, cerium and beryllium. In October 1945 Iowa State College received the Army-Navy "E" Award for Excellence in Production, an award usually only given to industrial organizations. In 1947 it became the Ames Laboratory, a national laboratory under the Atomic Energy Commission.

Compounds of thorium any chemical compound having at least one atom of thorium

Many compounds of thorium are known: this is because thorium and uranium are the most stable and accessible actinides and are the only actinides that can be studied safely and legally in bulk in a normal laboratory. As such, they have the best-known chemistry of the actinides, along with that of plutonium, as the self-heating and radiation from them is not enough to cause radiolysis of chemical bonds as it is for the other actinides. While the later actinides from americium onwards are predominantly trivalent and behave more similarly to the corresponding lanthanides, as one would expect from periodic trends, the early actinides up to plutonium have relativistically destabilised and hence delocalised 5f and 6d electrons that participate in chemistry in a similar way to the early transition metals of group 3 through 8: thus, all their valence electrons can participate in chemical reactions, although this is not common for neptunium and plutonium.


  1. 1 2 3 4 5 6 7 Haynes, p. 4.95
  2. Haynes, p. 4.136
  3. Haynes, p. 4.144
  4. 1 2 Yamashita, Toshiyuki; Nitani, Noriko; Tsuji, Toshihide; Inagaki, Hironitsu (1997). "Thermal expansions of NpO2 and some other actinide dioxides". J. Nucl. Mater. 245 (1): 72–78. Bibcode:1997JNuM..245...72Y. doi:10.1016/S0022-3115(96)00750-7.
  5. Emsley, John (2001). Nature's Building Blocks (Hardcover, First ed.). Oxford University Press. pp.  441. ISBN   978-0-19-850340-8.
  6. He, Heming; Majewski, Jaroslaw; Allred, David D.; Wang, Peng; Wen, Xiaodong; Rector, Kirk D. (2017). "Formation of solid thorium monoxide at near-ambient conditions as observed by neutron reflectometry and interpreted by screened hybrid functional calculations". Journal of Nuclear Materials. 487: 288–296. Bibcode:2017JNuM..487..288H. doi: 10.1016/j.jnucmat.2016.12.046 .
  7. Hoch, Michael; Johnston, Herrick L. (1954). "The Reaction Occurring on Thoriated Cathodes". J. Am. Chem. Soc. 76 (19): 4833–4835. doi:10.1021/ja01648a018.
  8. Mitchell, Brian S (2004). An Introduction to Materials Engineering. and Science for Chemical and Materials. p. 473. ISBN   978-0-471-43623-2.
  9. Robertson, Wayne M. (1979). "Measurement and evaluation of hydrogen trapping in thoria dispersed nickel". Metallurgical and Materials Transactions A. 10 (4): 489–501. Bibcode:1979MTA....10..489R. doi:10.1007/BF02697077.
  10. Kumar, Arun; Nasrallah, M.; Douglass, D. L. (1974). "The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys". Oxidation of Metals. 8 (4): 227–263. doi:10.1007/BF00604042. hdl: 2060/19740015001 . ISSN   0030-770X.
  11. Stringer, J.; Wilcox, B. A.; Jaffee, R. I. (1972). "The high-temperature oxidation of nickel-20 wt.% chromium alloys containing dispersed oxide phases". Oxidation of Metals. 5 (1): 11–47. doi:10.1007/BF00614617. ISSN   0030-770X.
  12. Murr, L. E. (1974). "Interfacial energetics in the TD-nickel and TD-nichrome systems". Journal of Materials Science. 9 (8): 1309–1319. doi:10.1007/BF00551849. ISSN   0022-2461.
  13. Stoll, Wolfgang (2012) "Thorium and Thorium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi : 10.1002/14356007.a27_001
  14. Thorotrast.
  15. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 1425, 1456. ISBN   978-0-08-022057-4.Cite has empty unknown parameter: |1= (help)
  16. Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC Press. ISBN   978-0-8493-0485-9.
  17. Oak Ridge Associated Universities (1999). "Thoriated Camera Lens (ca. 1970s)" . Retrieved 29 September 2017.
  18. Stoll, W. (2005). "Thorium and Thorium Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. p. 32. doi:10.1002/14356007.a27_001. ISBN   978-3-527-31097-5.

Cited sources