Uranium-dioxide

Lightwater reactors, the primary type of nuclear power reactor operated throughout the world, are fueled with uranium dioxide [1344-57-6] UO2 miched from the naturally occurring concentration of 0.71% uranium-235 [15117-96-17, to approximately 3% (1). As of this writing all civiUan nuclear... 

Several components are required in the practical appHcation of nuclear reactors (1 5). The first and most vital component of a nuclear reactor is the fuel, which is usually uranium slightly enriched in uranium-235 [15117-96-1] to approximately 3%, in contrast to natural uranium which has 0.72% Less commonly, reactors are fueled with plutonium produced by neutron absorption in uranium-238 [24678-82-8]. Even more rare are reactors fueled with uranium-233 [13968-55-3] produced by neutron absorption in thorium-232 (see Nuclear reactors, nuclear fuel reserves). The chemical form of the reactor fuel typically is uranium dioxide, UO2, but uranium metal and other compounds have been used, including sulfates, siUcides, nitrates, carbides, and molten salts. 

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

Uranium dioxide fuel is irradiated in a reactor for periods of one to two years to produce fission energy. Upon removal, the used or spent fuel contains a large inventory of fission products. These are largely contained in the oxide matrix and the sealed fuel tubing. 

Uranium [7440-61-17 is a naturally occurring radioactive element with atomic number 92 and atomic mass 238.03. Uranium was discovered in a pitchblende [1317-75-5] specimen ia 1789 by M. H. Klaproth (1) who named the element uranit after the planet Uranus, which had been recendy discovered. For 50 years the material discovered by Klaproth was thought to be metallic uranium. Pnligot showed that the uranit discovered by Klaproth was really uranium dioxide [1344-57-6] UO2, and obtained the tme elemental uranium as a black powder in 1841 by reduction of UCl [10026-10-5] with potassium (2). 

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

Uranium dioxide [1344-57-6], UO2, is found ki nature as the mineral pitchblende and as a component ki uraninite. The crystalline soHd melts at 2878°C and is paramagnetic with a room temperature magnetic moment of 3.2 )Xg. The density has been found to range from 10.79 to 10.95 g/cm, lower values are... 

The main technological uses for UO2 are found in the nuclear fuel cycle as the principal component for light and heavy water reactor fuels. Uranium dioxide is also a starting material for the synthesis of UF [10049-14-6] 6 (both critical for the production of pure uranium metal and... 

J. K. Fink, Tables of Thermodynamic andTransport Properties of Uranium Dioxide, 1982. 

Carbides of the Actinides, Uranium, and Thorium. The carbides of uranium and thorium are used as nuclear fuels and breeder materials for gas-cooled, graphite-moderated reactors (see Nuclearreactors). The actinide carbides are prepared by the reaction of metal or metal hydride powders with carbon or preferably by the reduction of the oxides uranium dioxide [1344-57-6] UO2 tduranium octaoxide [1344-59-8], U Og, or thorium... 

Fig. 5.8. (a) Packing of the unequally sized ions of sodium chloride to give a f.c.c. structure KCl and MgO pack in the same v/ay. (b) Packing of ions in uranium dioxide this is more complicated than in NaCi because the U and O ions are not in 1 1 ratio. 

Nuclear power is now the only substantial use for uranium. But before uranium can be used in a nuclear reactor, it must undergo several processes. After uranium is mined from geological mineral deposits, it is purified and converted into uranium hexafluoride (UF,). The UF, is next enriched, increasing the concentration of U-235 by separating out UF,5 made with U-238 atoms. The enriched UF, is then converted into uranium dioxide (UO,), and pressed into fuel pellets for use in the nuclear reactor. 

Since the uranium from the milling process is still in an unusable form, the yellow cake is broken down once again. The uranium trioxide is reduced to uranium dioxide at veiy high temperatures. Refining of the product also takes place. Now the uranium product consists almost entirely of UO,. 

Uranium tarnishes readily in the atmosphere at room temperature. Electropolishing inhibits the process whilst etching in nitric acid activates the surface. Uranium dioxide and hydrated UO3 are the principal solid products. 

The solid corrosion products in carbon dioxide and carbon monoxide are uranium dioxide, uranium carbides and carbon. The major reaction with carbon dioxide results in the formation of carbon monoxide ... 

Roles of Niobium Pentoxide, Vanadium Pentoxide and Titanium Dioxide In The Grain Growth And Sintering of Uranium Dioxide ,... 

One of the most Important thermophysical properties of reactor fuel In reactor safety analysis Is vapor pressure, for which data are needed for temperatures above 3000 K. We have recently completed an analysis of the vapor pressure and vapor composition In equilibrium with the hypostolchiometric uranium dioxide condensed phase (1 ), and we present here a similar analysis for the plutonium/oxygen (Pu/0) system. 

Uranium dioxide, UQ2, can be further oxidized to give a nonstoichiometric compound U024v, where 0 < x < 0.25. See Exercise 5.77 for a description of nonstoichiometric compounds, (a) What is the average oxidation state of uranium in a compound with composition UO,j-> (b) If we assume that the uranium exists in either the +4 or the +5 oxidation state, what is the fraction of uranium ions in each ... 

One of the most important examples of the fluorination of oxides is the fluorination of uranium dioxide. Uranium tetrafluoride (UF4) is the intermediate compound which is reduced to uranium metal. The gaseous higher fluoride, uranium hexafluoride (UF6) is used for the separation of uranium isotopes to obtain enriched uranium (i.e., uranium containing a higher proportion of the isotope, U235, than natural uranium). 

Another, more modern, route of processing the yellow cake is shown in Figure 5.38, accomplishes the production of enriched uranium oxide entirely by pyroprocessing. Thus, uranium is finally obtained in three forms metallic uranium, enriched uranium dioxide, and natural uranium dioxide. As the flowsheet shows, and as briefly described herein, these are essentially the products of hydro and pyro-based processing schemes. 

Table 4 summarizes the geographical behavior of fission products at the Oklo reactors compared with the behavior of fission products from neutron-irradiated uranium dioxide. The reactor zones RZ 1-9 are exposed on the ground, while RZ 10-16 are situated below the ground, all quite deep except RZ 15. 

Table 4 Comparison of the characteristic behavior of fission products in the Oklo ores with those in irradiated uranium dioxide...

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