uranium dioxide

Uranium dioxide is produced by reducing uranium trioxide with hydrogen. This reaction often creates triuranium octoxide as an intermediate.{{cite journal |date=1974 |title=The kinetics of hydrogen reduction of UO3 and U3O8 derived from ammonium diuranate |author1=A.H. Le Page |author2=A.G. Fane |journal=Journal of Inorganic and Nuclear Chemistry |doi=10.1016/0022-1902(74)80663-9 |volume=36 |issue=1 |pages=87–92}}{{cite journal |date=1 July 1962 |title=Hydrogen Reduction of Uranium Oxides. A Phase Study by Means of a Controlled-Atmosphere Diffractometer Hot Stage |author1=Notz, K.J. |author2=Huntington, C.W. |author3=Burkhardt, W. |journal=Industrial & Engineering Chemistry Process Design and Development |doi=10.1021/i260003a010 |volume=1 |issue=3 |pages=213–217}}{{cite web |url=https://world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/conversion-and-deconversion |title=Conversion and Deconversion |date=20 Nov 2024 |website=World Nuclear Association |access-date=18 Mar 2025}}

:UO₃ + H₂ → UO₂ + H₂O at 700 °C (973 K)

This reaction plays an important part in the creation of nuclear fuel through nuclear reprocessing and uranium enrichment.

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, thorium, and the transuranic elements from neptunium through californium have the same structures.{{Cite journal |last1=Petit |first1=L. |last2=Svane |first2=A. |last3=Szotek |first3=Z. |last4=Temmerman |first4=W. M. |last5=Stocks |first5=G. M. |date=2010-01-07 |title=Electronic structure and ionicity of actinide oxides from first principles |url=https://link.aps.org/doi/10.1103/PhysRevB.81.045108 |journal=Physical Review B |volume=81 |issue=4 |pages=045108 |doi=10.1103/PhysRevB.81.045108|arxiv=0908.1806 |bibcode=2010PhRvB..81d5108P |s2cid=118365366 }} 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.{{cite journal |doi=10.1126/science.1259709 |pmid=25414311 |title=Molten uranium dioxide structure and dynamics |journal=Science |volume=346 |issue=6212 |pages=984–7 |year=2014 |last1=Skinner |first1=L. B. |last2=Benmore |first2=C. J. |last3=Weber |first3=J. K. R. |last4=Williamson |first4=M. A. |last5=Tamalonis |first5=A. |last6=Hebden |first6=A. |last7=Wiencek |first7=T. |last8=Alderman |first8=O. L. G. |last9=Guthrie |first9=M. |last10=Leibowitz |first10=L. |last11=Parise |first11=J. B. |bibcode=2014Sci...346..984S |osti=1174101 |s2cid=206561628 |url=https://www.osti.gov/biblio/1174101 }} Models consistent with these measurements show the melt to consist mainly of UO₆ and UO₇ polyhedral units, where roughly {{frac|2|3}} of the connections between polyhedra are corner sharing and {{frac|1|3}} are edge sharing.

UO2 Powder.jpg|Uranium dioxide

UO2 Pellet.jpg|Sintered uranium dioxide pellet

Uranium dioxide is oxidized in contact with oxygen to form triuranium octoxide:{{cite journal |date=2006 |title=A detailed study of UO2 to U3O8 oxidation phases and the associated rate-limiting steps |author1=G. Rousseau |author2=L. Desgranges |author3=F. Charlot |author4=N. Millot |author5=J.C. Nièpce |author6=M. Pijolat |author7=F. Valdivieso |author8=G. Baldinozzi |author9=J.F. Bérar |journal=Journal of Nuclear Materials |doi=10.1016/j.jnucmat.2006.03.015 |volume=355 |issue=1–3 |pages=10–20}}

:3 UO₂ + O₂ → U₃O₈ at 250 °C (523 K)

The electrochemistry of uranium dioxide has been investigated in detail as the galvanic corrosion of uranium dioxide controls the rate at which used nuclear fuel dissolves.{{clarify|date=March 2025}} See spent nuclear fuel for further details. Water increases the oxidation rate of plutonium and uranium metals.{{cite journal |doi=10.1016/S0925-8388(00)01222-6 |title=Reactions of plutonium dioxide with water and hydrogen–oxygen mixtures: Mechanisms for corrosion of uranium and plutonium |journal=Journal of Alloys and Compounds |volume=314 |issue=1–2 |pages=78–91 |year=2001 |last1=Haschke |first1=John M |last2=Allen |first2=Thomas H |last3=Morales |first3=Luis A }}

Uranium dioxide reacts with carbon at high temperatures, forming uranium carbide and carbon monoxide.{{cite thesis |last=Buchel |first=Gerald L. |date=1963 |title=Equilibrium studies of the uranium dioxide - carbon and neodymium sesquioxide - carbon systems |degree=M.S. |publisher=Michigan State University}}

:{{chem2 | UO2 + 4 C -> UC2 + 2 CO }}

This process must be done under an inert gas as uranium carbide is easily oxidized back into uranium oxide.

UO₂ is used mainly as nuclear fuel, 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 reactors.{{cite web |url=https://world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/mixed-oxide-fuel-mox |title=Mixed Oxide (MOX) Fuel |date=10 Oct 2017 |website=World Nuclear Association |access-date=11 Mar 2025}}

The thermal conductivity of uranium dioxide is very low when compared with elemental uranium, uranium nitride, uranium carbide and zircaloy cladding material as well as most uranium-based alloys.{{cite journal |date=2024 |title=Incorporation of uranium nitride fuel capability into the ENIGMA fuel performance code: Model development and validation |last1=Peakman |first1=Aiden |last2=Rossiter |first2=Glyn |journal=Nuclear Engineering and Design |doi=10.1016/j.nucengdes.2024.113604 |volume=429 |pages=113604 |bibcode=2024NuEnD.42913604P |issn=0029-5493|doi-access=free }}{{cite web |author=Dr. Celine Hin |title=Thermal Conductivity of Metallic Uranium |website=U.S. Department of Energy Office of Scientific and Technical Information |url=https://www.osti.gov/servlets/purl/1433931 |access-date=18 Mar 2025}}{{cite web |author=R. R. Hammer |date=September 1967 |title=Zircaloy-4, Uranium Dioxide, and Materials Formed by their Interaction: A Literature Review with Extrapolation of Physical Properties to High Temperatures |website=U.S. Department of Energy Office of Scientific and Technical Information |url=https://www.osti.gov/servlets/purl/4136888 |access-date=18 Mar 2025 |publisher=Idaho Nuclear Corporation}} This low thermal conductivity can result in localised overheating in the centres of fuel pellets.{{cite web |url=https://www.nuclear-power.com/nuclear-engineering/heat-transfer/thermal-conduction/thermal-conductivity/thermal-conductivity-of-uranium-dioxide/ |title=Thermal Conductivity of Uranium Dioxide |author= |website=Nuclear Power |access-date=18 Mar 2025}}

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.{{citation needed|date=January 2017}}

file:ZrUthermalcond.png

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

File:FiestaWare Velleman-K2645 GeigerCounter.ogv

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.{{Cite book|url=http://www.uranglasuren.com/|title=Uran in der Keramik. Geschichte - Technik - Hersteller|last=Örtel|first=Stefan}} Orange-colored Fiestaware is a well-known example of a product with a urania-colored glaze.Radon, Health and Natural Hazards, Editors: G.K. Gillmore, F.E. Perrier, R.G.M. Crockett, pp. 50-52, 2018, Geological Society of London, {{ISBN|1786203081}}, 9781786203083, [https://books.google.com/books?id=oJtTDwAAQBAJ&pg=PA52 Google Books] Uranium glass is pale green to yellow and often has strong fluorescent properties.{{cite web |url=https://www.orau.org/blog/museum/a-glowing-review-of-uranium-glass.html |title=A 'glowing' review of uranium glass |author=Amber Davis |publisher=Oak Ridge Associated Universities |date=22 Jan 2025 |website=ORAU: Then & Now}} Urania has also been used in formulations of enamel and porcelain.{{cite journal |date=19 Jul 2013 |title=Uranium in glass, glazes and enamels: history, identification and handling} |author=Donna Strahan |journal=Studies in Conservation |doi=10.1179/sic.2001.46.3.181 |volume=46 |issue=3 |pages=181–195}} It is possible to determine with a Geiger counter if a glaze or glass produced before 1958 contains urania.

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.{{cite journal |date=2012 |title=Uranium: A Dentist's perspective |author1=Toor, R. S. S. |author2=Brar, G. S. |journal=Journal of International Society of Preventive and Community Dentistry |doi=10.4103/2231-0762.103447 |volume=2 |issue=1 |pages=1–7|doi-access=free |pmid=24478959 |pmc=3894091 }}

Depleted UO₂ (DUO₂) can be used as a material for radiation shielding. For example, DUCRETE is a "heavy concrete" material where gravel is replaced with uranium dioxide aggregate; this material is investigated for use for casks for radioactive waste.{{cite report |last=Lessing |first=Paul A. |date=1 Mar 1995 |title=Development of "DUCRETE" |publisher=Idaho National Engineering Laboratory |doi=10.2172/366558 |osti=366558 |url=https://digital.library.unt.edu/ark:/67531/metadc676180/ }} Casks can be also made of DUO₂-steel cermet, a composite material made of an aggregate of uranium dioxide serving as radiation shielding, graphite 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.{{cite document |last1=Forsberg |first1=Charles W. |last2=Swaney |first2=Paul M. |last3=Tiegs |first3=Terry N. |title=Characteristics and Fabrication of Cermet Spent Nuclear Fuel Casks: Ceramic Particles Embedded in Steel |type=Conference |publisher=14th International Symposium on the Packaging and Transportation of Radioactive Materials (PATRAM 2004)}} https://www.osti.gov/etdeweb/servlets/purl/20773271

Depleted uranium dioxide can be also used as a catalyst, e.g. for degradation of volatile organic compounds in gaseous phase, oxidation of methane to methanol, and removal of sulfur from petroleum. It has high efficiency and long-term stability when used to destroy VOCs when compared with some of the commercial catalysts, 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.{{cite journal |doi=10.1038/384341a0 |title=Uranium-oxide-based catalysts for the destruction of volatile chloro-organic compounds |journal=Nature |volume=384 |issue=6607 |pages=341–3 |year=1996 |last1=Hutchings |first1=Graham J. |last2=Heneghan |first2=Catherine S. |last3=Hudson |first3=Ian D. |last4=Taylor |first4=Stuart H. |bibcode=1996Natur.384..341H |s2cid=4299921 }}

The use of uranium dioxide as a material for rechargeable batteries is being investigated.{{cite journal |date=10 Jun 2013 |title=From Used Oxide Nuclear Fuel to Rechargeable Battery: A First-Principles Study |last1=Wu |first1=Binbin |last2=Yu |first2=Jianguo |journal=MRS Proceedings |doi=10.1557/opl.2013.732 |volume=1541}} The batteries could have a high power density and a reduction potential of -4.7 V per cell.{{cite web |title=Initial Development of a Depleted Uranium Battery |last1=Dunbar |first1=Paul |last2=Lee-Desautels |first2=Rhonda |publisher=University of Kentucky |url=https://scholars.uky.edu/en/projects/initial-development-of-a-depleted-uranium-battery}} Another investigated application is in photoelectrochemical cells 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.{{citation needed|date=January 2017}}

Uranium dioxide displays strong piezomagnetism in the antiferromagnetic state, observed at cryogenic temperatures below 30 kelvins. 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.{{cite journal |doi=10.1038/s41467-017-00096-4 |title=Piezomagnetism and magnetoelastic memory in uranium dioxide. |journal=Nature Communications |volume=8 |pages=99 |year=2017 |last1=Jaime |first1=Marcelo |last2=Saul |first2=Andres |last3=Salamon |first3=Myron B. |last4=Zapf |first4=Vivien |last5=Harrison |first5=Neil |last6=Durakiewicz |first6=Tomasz |last7=Lashley |first7=Jason C. |last8=Andersson |first8=David A. |last9=Stanek |first9=Christopher R. |last10=Smith |first10=James L. |last11=Gofryk |first11=Krysztof|issue=1 |pmid=28740123 |pmc=5524652 |bibcode=2017NatCo...8...99J }} 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.{{cite journal |doi=10.1038/s43246-021-00121-6 |bibcode=2021CoMat...2...17A |title=Piezomagnetic switching and complex phase equilibria in uranium dioxide. |journal=Communications Materials |volume=2 |issue=1 |pages=17 |year=2021 |last1=Antonio |first1=Daniel J. |last2=Weiss |first2=Joel T. |last3=Shanks |first3=Katherine S. |last4=Ruff |first4=Jacob P.C. |last5=Jaime |first5=Marcelo |last6=Saul |first6=Andres |last7=Swinburne |first7=Thomas |last8=Salamon |first8=Myron B. |last9=Lavina |first9=Barbara |last10=Koury |first10=Daniel |last11=Gruner |first11=Sol M. |last12=Andersson |first12=David A. |last13=Stanek |first13=Christopher R. |last14=Durakiewicz |first14=Tomasz |last15=Smith |first15=James L. |last16=Islam |first16=Zahir |last17=Gofryk |first17=Krysztof|arxiv=2104.06340 |s2cid=231812027 }}

The band gap of uranium dioxide is comparable to those of silicon and gallium arsenide, near the optimum for efficiency vs band gap curve for absorption of solar radiation, suggesting its possible use for very efficient solar cells based on Schottky diode 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.{{cite journal |doi=10.1103/PhysRevLett.106.207402 |pmid=21668262 |title=Ultrafast Hopping Dynamics of 5f Electrons in the Mott Insulator UO₂ Studied by Femtosecond Pump-Probe Spectroscopy |journal=Physical Review Letters |volume=106 |issue=20 |pages=207402 |year=2011 |last1=An |first1=Yong Q. |last2=Taylor |first2=Antoinette J.|author2-link=Antoinette Taylor |last3=Conradson |first3=Steven D. |last4=Trugman |first4=Stuart A. |last5=Durakiewicz |first5=Tomasz |last6=Rodriguez |first6=George |bibcode=2011PhRvL.106t7402A }}

The dielectric constant of uranium dioxide is about 21.5,{{cite journal |date=1 Sep 1987 |title=The pressure dependence of the dielectric constant and electrical conductivity of single crystal uranium dioxide |first1=R. N. |last1=Hampton |first2=G. A. |last2=Saunders |first3=J. H. |last3=Harding |first4=A. M. |last4=Stoneham |journal=Journal of Nuclear Materials |doi=10.1016/0022-3115(87)90089-4 |volume=150 |issue=1 |pages=17–23|bibcode=1987JNuM..150...17H }} which is almost twice as high as of silicon (11.7){{cite journal |date=Aug 2019 |title=Permittivity of Undoped Silicon in the Millimeter Wave Range |first1=Xiaofan |last1=Yang |first2=Xiaoming |last2=Liu |first3=Shuo |last3=Yu |first4=Lu |last4=Gan |first5=Jun |last5=Zhou |first6=Yonghu |last6=Zeng |journal=Electronics |doi=10.3390/electronics8080886 |volume=8 |issue=8 |pages=886|doi-access=free }} and GaAs (12.4).{{cite book |page=283 |last=Fox |first=Mark |date=2010 |title=Optical Properties of Solids |edition=2 |url=https://global.oup.com/academic/product/optical-properties-of-solids-9780199573370?lang=en&cc=no |publisher=Oxford University Press |isbn=978-0199573370 }} This is an advantage over Si and GaAs in the construction of integrated circuits, as it may allow higher density integration with higher breakdown voltages and with lower susceptibility to the CMOS tunnelling breakdown.{{cite conference |title=Semiconductive Properties of Uranium Oxides |first1=Thomas |last1=Meek |first2=Michael |last2=Hu |first3=M. Jonathan |last3=Haire |format=DOC |conference=Waste Management Symposium 2001 |date=25 Feb – 1 Mar 2001 |location=Tucson |url=https://archivedproceedings.econference.io/wmsym/2001/14/14-3.pdf |access-date=13 Apr 2025}}

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 the −170 μV/K (n-type) / 160 μV/K (p-type) of bismuth telluride,{{cite journal |date=28 Mar 2014 |title=Bismuth Telluride and Its Alloys as Materials for Thermoelectric Generation |last=Goldsmid |first=H. Julian |journal=Materials |doi=10.3390/ma7042577 |volume=7 |issue=4 |pages=2577–2592 |doi-access=free |pmid=28788584|pmc=5453363 |bibcode=2014Mate....7.2577G }} other materials promising for thermoelectric power generation applications and Peltier elements.{{citation needed|date=April 2025}}

The radioactive decay impact of the ²³⁵U and ²³⁸U on its semiconducting properties was not measured {{as of|2005|lc=on}}. 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 high-performance integrated circuits. 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 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 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.{{citation needed|date=April 2025}}

Uranium dioxide is also resistant to radiation damage, making it useful for rad-hard devices{{citation needed|date=April 2025}} for special military and aerospace applications.

A Schottky diode of U₃O₈ and a p-n-p transistor of UO₂ were successfully manufactured in a laboratory.{{cite journal |doi=10.1016/j.vacuum.2008.04.005 |title=Semiconductor devices fabricated from actinide oxides |journal=Vacuum |volume=83 |issue=1 |pages=226–8 |year=2008 |last1=Meek |first1=Thomas T. |last2=von Roedern |first2=B. |bibcode=2008Vacuu..83..226M }}

Uranium dioxide is known to be absorbed by phagocytosis in the lungs.Principles of Biochemical Toxicology. Timbrell, John. PA 2008 {{ISBN|0-8493-7302-6}}{{page needed|date=January 2017}}

• Cleveite
• Ducrete
• Uranium oxide
• Uranium glass
• Uranium tile

{{reflist}}

• {{cite journal |doi=10.1107/S0567740882009935 |title=The preparation and structure of barium uranium oxide BaUO3+x |journal=Acta Crystallographica Section B |volume=38 |issue=11 |pages=2775 |year=1982 |last1=Barrett |first1=S. A. |last2=Jacobson |first2=A. J. |last3=Tofield |first3=B. C. |last4=Fender |first4=B. E. F. |bibcode=1982AcCrB..38.2775B }}

• [http://web.ead.anl.gov/uranium/pdf/WM01Semicond.pdf Semiconducting properties of uranium oxides] {{Webarchive|url=https://web.archive.org/web/20120901192557/http://web.ead.anl.gov/uranium/pdf/WM01Semicond.pdf |date=2012-09-01 }}
• [http://www.thefreedictionary.com/uranium+dioxide Free Dictionary Listing for Uranium Dioxide]
• The [http://www.ibilabs.com/Uranium%20Oxide,%20di.htm Uranium dioxide] {{Webarchive|url=https://web.archive.org/web/20130916230607/http://ibilabs.com/Uranium%20Oxide,%20di.htm |date=2013-09-16 }} International Bio-Analytical Industries, Inc.

{{Uranium compounds}}

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