Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of

uranium 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 weak ...

, and is a black,

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

line powder that naturally occurs in the mineral uraninite. It is used in

nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergo ...

rods in nuclear reactors. A mixture of uranium and

plutonium 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 exh ...

dioxides is used as MOX fuel. Prior to 1960, it was used as yellow and black color in ceramic glazes and glass.

Production

Uranium dioxide is produced by reducing uranium trioxide with

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

. :UO₃ + H₂ → UO₂ + H₂O at 700 °C (973 K) This reaction plays an important part in the creation of

through

nuclear reprocessing Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, th ...

and uranium enrichment.

Chemistry

Structure

The solid is isostructural with (has the same structure as) fluorite ( calcium fluoride), where each U is surrounded by eight O nearest neighbors in a cubic arrangement. In addition, the dioxides of

cerium Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 ...

, thorium, and the transuranic elements from neptunium through

californium Californium is a radioactive chemical element with the symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding c ...

have the same structures. No other elemental dioxides have the fluorite structure. Upon melting, the measured average U-O coordination reduces from 8 in the crystalline solid (UO₈ cubes), down to 6.7±0.5 (at 3270 K) in the melt. Models consistent with these measurements show the melt to consist mainly of UO₆ and UO₇ polyhedral units, where roughly of the connections between polyhedra are corner sharing and are edge sharing. UO2 Powder.jpg, Uranium dioxide UO2 Pellet.jpg, Sintered uranium dioxide pellet

Oxidation

Uranium dioxide is

oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

in contact with

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

to the triuranium octaoxide. :3 UO₂ + O₂ → U₃O₈ at 700 °C (970 K) The

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...

of uranium dioxide has been investigated in detail as the galvanic corrosion of uranium dioxide controls the rate at which used

dissolves. See spent nuclear fuel for further details.

Water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

increases the oxidation rate of

and

metals.

Carbonization

Uranium dioxide is carbonized in contact with

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

, forming uranium carbide and

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

. :UO2 \ + \ 4C -> UC2 \ + \ 2CO. This process must be done under an

inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

as uranium carbide is easily oxidized back into uranium oxide.

Uses

Nuclear fuel

UO₂ is used mainly as

, specifically as UO₂ or as a mixture of UO₂ and PuO₂ ( plutonium dioxide) called a mixed oxide ( MOX fuel), in the form of fuel rods in

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

s. Note that the

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of uranium dioxide is very low when compared with

, uranium nitride, uranium carbide and zirconium cladding material. This low thermal conductivity can result in localised overheating in the centres of fuel pellets. The graph below shows the different temperature gradients in different fuel compounds. For these fuels, the thermal power density is the same and the diameter of all the pellets are the same. FuelPellet1.jpg, Uranium oxide fuel pellet RIAN archive 132609 Uranium dioxide fuel pellet starting material.jpg, Starting material containers for uranium dioxide fuel pellet production at a plant in Russia ZrUthermalcond.png, The thermal conductivity of zirconium metal and uranium dioxide as a function of temperature

Color for glass ceramic glaze

Uranium oxide (urania) was used to color glass and ceramics prior to World War II, and until the applications of radioactivity were discovered this was its main use. In 1958 the military in both the US and Europe allowed its commercial use again as depleted uranium, and its use began again on a more limited scale. Urania-based ceramic glazes are dark green or black when fired in a reduction or when UO₂ is used; more commonly it is used in oxidation to produce bright yellow, orange and red glazes. Orange-colored

Fiestaware Fiesta is a line of ceramic glazed dinnerware manufactured and marketed by the Fiesta Tableware Company of Newell, West Virginia since its introduction in 1936, with a hiatus from 1973 to 1985. Fiesta is noted for its Art Deco styling and its ...

is a well-known example of a product with a urania-colored glaze. Uranium glass is pale green to yellow and often has strong fluorescent properties. Urania has also been used in formulations of enamel and

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...

. It is possible to determine with a Geiger counter if a glaze or glass produced before 1958 contains urania.

Other uses

Prior to the realisation of the harmfulness of radiation, uranium was included in false teeth and dentures, as its slight fluorescence made the dentures appear more like real teeth in a variety of lighting conditions. Depleted UO₂ (DUO₂) can be used as a material for radiation shielding. For example,

DUCRETE DUCRETE (Depleted Uranium Concrete) is a high density concrete alternative investigated for use in construction of casks for storage of radioactive waste. It is a composite material containing depleted uranium dioxide aggregate instead of convent ...

is a "heavy

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

" material where gravel is replaced with uranium dioxide aggregate; this material is investigated for use for casks for

radioactive waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapon ...

. Casks can be also made of DUO₂-

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...

cermet, a

composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...

made of an

aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...

of uranium dioxide serving as radiation shielding,

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...

and/or silicon carbide serving as neutron radiation absorber and moderator, and steel as the matrix, whose high thermal conductivity allows easy removal of decay heat. Depleted uranium dioxide can be also used as a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

, e.g. for degradation of volatile organic compounds in gaseous phase,

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

, and removal of

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

from

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

. It has high efficiency and long-term stability when used to destroy VOCs when compared with some of the commercial

s, such as precious metals, TiO₂, and Co₃O₄ catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity. The use of uranium dioxide as a material for rechargeable batteries is being investigated. The batteries could have high

power density Power density is the amount of power (time rate of energy transfer) per unit volume. In energy transformers including batteries, fuel cells, motors, power supply A power supply is an electrical device that supplies electric power to a ...

and potential of 4.7 V per cell. Another investigated application is in

photoelectrochemical cell A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelec ...

s for solar-assisted hydrogen production where UO₂ is used as a photoanode. In earlier times, uranium dioxide was also used as heat conductor for current limitation (URDOX-resistor), which was the first use of its semiconductor properties. Uranium dioxide displays strong piezomagnetism in the antiferromagnetic state, observed at cryogenic temperatures below 30

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...

s. Accordingly, the linear magnetostriction found in UO₂ changes sign with the applied magnetic field and exhibits magnetoelastic memory switching phenomena at record high switch-fields of 180,000 Oe. The microscopic origin of the material magnetic properties lays in the face-centered-cubic crystal lattice symmetry of uranium atoms, and its response to applied magnetic fields.

Semiconductor properties

The band gap of uranium dioxide is comparable to those of

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

and

gallium arsenide Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated c ...

, near the optimum for efficiency vs band gap curve for absorption of solar radiation, suggesting its possible use for very efficient

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

s based on

Schottky diode The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage ...

structure; it also absorbs at five different wavelengths, including infrared, further enhancing its efficiency. Its intrinsic conductivity at room temperature is about the same as of single crystal silicon. The

dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insula ...

of uranium dioxide is about 22, which is almost twice as high as of silicon (11.2) and GaAs (14.1). This is an advantage over Si and GaAs in the construction of

integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

s, as it may allow higher density integration with higher breakdown voltages and with lower susceptibility to the

CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...

tunnelling breakdown. The Seebeck coefficient of uranium dioxide at room temperature is about 750 µV/K, a value significantly higher than the 270 µV/K of thallium tin telluride (Tl₂SnTe₅) and thallium germanium telluride (Tl₂GeTe₅) and of bismuth-

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...

alloys, other materials promising for thermoelectric power generation applications and Peltier elements. The radioactive decay impact of the ²³⁵U and ²³⁸U on its semiconducting properties was not measured . Due to the slow decay rate of these isotopes, it should not meaningfully influence the properties of uranium dioxide solar cells and thermoelectric devices, but it may become an important factor for VLSI chips. Use of depleted uranium oxide is necessary for this reason. The capture of alpha particles emitted during radioactive decay as helium atoms in the crystal lattice may also cause gradual long-term changes in its properties. The

stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...

of the material dramatically influences its electrical properties. For example, the electrical conductivity of UO_1.994 is orders of magnitude lower at higher temperatures than the conductivity of UO_2.001 . Uranium dioxide, like U₃O₈, is a

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...

material capable of withstanding high temperatures (about 2300 °C, in comparison with at most 200 °C for silicon or GaAs), making it suitable for high-temperature applications like thermophotovoltaic devices. Uranium dioxide is also resistant to

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

damage, making it useful for rad-hard devices for special military and

aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and ast ...

applications. A

of U₃O₈ and a p-n-p transistor of UO₂ were successfully manufactured in a laboratory.

Toxicity

Uranium dioxide is known to be absorbed by phagocytosis in the lungs.Principles of Biochemical Toxicology. Timbrell, John. PA 2008

References

External links

Semiconducting properties of uranium oxides

Free Dictionary Listing for Uranium Dioxide
* The
/span> International Bio-Analytical Industries, Inc. {{DEFAULTSORT:Uranium Dioxide Nuclear chemistry Uranium(IV) compounds Nuclear materials Oxides Semiconductor materials Articles containing video clips Fluorite crystal structure