{"id":27461,"date":"2020-07-09T08:10:32","date_gmt":"2020-07-09T08:10:32","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=27461"},"modified":"2023-08-02T06:21:17","modified_gmt":"2023-08-02T06:21:17","slug":"uranium-dioxide-uo2","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power-plant\/nuclear-fuel\/fuel-assembly\/fuel-pellets\/uranium-dioxide-uo2\/","title":{"rendered":"Uranium Dioxide – UO2"},"content":{"rendered":"
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Uranium dioxide is a ceramic refractory uranium compound, in many cases, used as a nuclear fuel. Most LWRs<\/a> use uranium fuel<\/strong>\u00a0in the form of uranium dioxide (chemically UO<\/strong>2<\/sub><\/strong>)<\/strong>. Uranium dioxide is a black semiconducting solid with very low thermal conductivity. On the other hand, uranium dioxide has a very high melting point and has well-known behavior.<\/p>\n

Uranium dioxide has a significantly lower density than uranium in metal form. Uranium dioxide has a density of 10.97 g\/cm<\/strong>3<\/sup><\/strong>, but this value may vary with fuel burnup because, at low burnup, densification of pellets can occur, and at higher burnup, swelling occurs.<\/p>\n

Thermal Conductivity of Uranium Dioxide<\/h2>\n

See also:\u00a0Thermal Conduction of Uranium Dioxide<\/a><\/p>\n

The\u00a0thermal conductivity\u00a0<\/strong>of\u00a0uranium dioxide<\/strong> is very low when compared with metal uranium, uranium nitride, uranium carbide, and zirconium cladding material. Thermal conductivity is one of the parameters determining the fuel centerline temperature<\/strong>. This low thermal conductivity can result in localized overheating in the fuel centerline; therefore, this overheating must be avoided. Overheating of the fuel is prevented by maintaining the steady state peak linear heat rate<\/strong>\u00a0(LHR) or the\u00a0Heat Flux Hot Channel Factor \u2013 FQ<\/sub>(z)<\/a> below the level at which fuel centerline melting occurs. Expansion of the fuel pellet upon centerline melting may cause the pellet to stress the cladding to the point of failure.<\/p>\n

Thermal conductivity<\/strong>\u00a0of solid UO2<\/sub> with a density of 95% is estimated by the following correlation [Klimenko; Zorin]:<\/p>\n

\"thermal<\/a><\/ins><\/ins><\/ins><\/p>\n

where \u03c4 = T\/1000. The uncertainty of this correlation is +10% in the range from 298.15 to 2000 K and +20% in the range from 2000 to 3120 K.<\/p>\n

\"Thermal<\/a><\/p>\n

Special reference: Thermal and Nuclear Power Plants\/Handbook ed. by A.V. Klimenko and V.M. Zorin. MEI Press, 2003.<\/p>\n

Special reference: Thermophysical Properties of Materials For Nuclear Engineering: A Tutorial and Collection of Data. IAEA-THPH, IAEA, Vienna, 2008. ISBN 978\u201392\u20130\u2013106508\u20137.<\/p>\n

Yellowcake – U3O8<\/h2>\n

Yellowcake<\/strong>, chemically\u00a0U3<\/sub>O8<\/sub><\/strong>, is a uranium concentrate powder recovered from leach solutions. Yellowcake<\/strong> is an intermediate step in the processing of uranium ores and nuclear fuel production. Typically, yellowcake can be obtained through the conventional milling and chemical processing of uranium ore forming a coarse powder. Yellowcake is a highly concentrated uranium powder<\/strong>; around\u00a075% of the material is made up of uranium, or 750 kg of uranium oxide per tonne.<\/p>\n

Before fuel fabrication, i.e., before conversion or enrichment, yellowcake is refined to obtain almost pure uranium in the form of triuranium octoxide (U3<\/sub>O8<\/sub>). Pure U3<\/sub>O8<\/sub>\u00a0is then used in the preparation of uranium fuel for nuclear reactors, for which triuranium octoxide powder may be sintered into fuel pellets (UO2<\/sub>) for use in fuel rods for pressurized heavy-water reactors. For other systems that use enriched uranium, uranium conversion and enrichment must follow this process.<\/p>\n

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<\/span>References:<\/div>
Nuclear and Reactor Physics:<\/strong>\n