Oxide fuels uranium dioxide

Fuel fabrication. The enriched UFg is either reduced to metallic uranium and machined to the appropriate shape, or oxidized to uranium dioxide and formed into pellets of ceramic uranium dioxide (UO2). The pellets are then stacked and sealed inside metal tubes that are mounted into special fuel assemblies ready for use in a nuclear reactor (DOE 1995b Uranium Institute 1996). 

Pulverization takes place by oxidation ofQthe uranium dioxide (UO2) with air at elevated temperatures ( 400UC) which expands the fuel volume if the oxidation is continued until U3O8 is obtained, a 30% volume expansion is achieved. The volume increase ruptures the cladding and pulverizes the fuel. Complete oxidation of UO2 to UoOg is not required to obtain sufficient volume expansion for pulverization. 

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. 

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... 

Malinin, G. V. et al., Russ. Chem. Rev., 1975, 44, 392-397 Thermal decomposition of metal oxides was reviewed. Some oxides (cobalt(II, III) oxide, copper(II) oxide, lead(II, IV) oxide, uranium dioxide, triuranium octaoxide) liberate quite a high proportion of atomic oxygen, with a correspondingly higher potential for oxidation of fuels than molecular oxygen. 

The biological half-time of uranium dioxide in human lungs (occupational exposure) at German fuel fabrication facilities was estimated to be 109 days. Body burden measurements of uranium taken from 12 people who handled uranium oxides for 5-15 years were used for this determination. Twice a year for 6 years, a urinalysis was conducted on workers exposed to uranium. In vivo lung counting was performed on the last day before and the first day after a holiday period. Levels of uranium in feces were measured during the first 3 days and the last 3 days of a holiday period and the first 3 days after the restart of work. For some employees, the levels of uranium in feces was measured during 3 days one-half year after the holiday period (Schieferdecker et al. 1985). 

Another important consideration is the problem of dissolving the mixed-oxide fuel for subsequent reprocessing and plutonium recovery after the irradiated mixed-oxide fuel has been discharged from the reactor. When plutonium dioxide is in solid solution with uranium dioxide at low concentrations, as in the case of plutonium created during the irradiation of uranium dioxide... 

Uranium Dioxide Production. The majority of the world s nuclear reactors are fueled with slightly enriched UOg prepared in the form of dense sintered pellets that are encapsulated in small bore tubes of zirconium alloy or stainless steel. Hex at the required enrichment(s) is produced specifically for a given reactor charge and the first process step with the UFg is to convert it to UOg having the desired ceramic-grade quality. This means an oxide which after granulation and pelleting can be sintered quickly and uniformly to pellets of near stoichiometric density. 

Similar products can be formed by reactions of liquid sodium with solid oxides, and some of these reactions are of technical importance. The failure of fuel element canning tubes causes the contact of uranium dioxide fuel with liquid sodium at high temperatures. The stoichiometric UO2 does not react with sodium at temperatures of 400 and 600 °C due to its very high thermodynamic stability. Non-stoichiometric compounds, however, which may contain some UO3 or U3O8, are able to react. In this case, the sodium uranate-V, Na3U04, is formed, a face-centered cubic cell with a = 0.474 nm The same product is formed at temperatures above 550 °C by the reaction... 

Uranium dioxide has a number of properties that make it suitable for a fuel. The crystal structure is the fluorite (CaF2) type, similar to that of calcia-stabilised zirconia, and is stable to temperatures in excess of 2000 °C. Because it is a ceramic oxide, the material is refractory, chemically inert and resistant to corrosion Enrichment does not change these features. The oxide powder is pressed into pellets and sintered to a density of about 95 % maximum by traditional ceramic processing technology but is carried out in conditions that minimise risks from radiation effects. The pellets are contained in zirconium alloy (zircaloy) containers, which are then introduced into the reactor. The moderator, which... 

A suitable fuel, used in the Galileo Jupiter explorer, which was finally destroyed in 2003, is the isotope Pu. This is an a-emitter, which provides about 0.5 Wg . The half-life is 87.4 years. The fuel is the solid oxide plutonium dioxide, Pu02. Chemically, it is similar to the uranium dioxide used in thermal reactors, and adopts the same fluorite (Cap2) crystal structure, similar to that of calcia-stabiUsed zirconia and UO2. This structure is inert chemically and stable up to the melting point of approximately 2500 °C. The oxide is pressed and sintered into pellets under conditions that lead to high density and low, but not zero, porosity. This is to ensure dimensional stability of the pellets over the lifetime of the spacecraft because, as Pu is an a-emitter, the resulting helium gas must be allowed to escape. 

The fuel in fast-breeder reactors is an oxide, as with a thermal reactor. The material chosen is a solid solution of uranium and plutonium dioxides, U tPU -jC)2. This material shares the same fluorite (Cap2) structure-type as uranium dioxide and plutonium dioxide. 

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide UjO (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a ceramic, which is the most common fuel material used in commercial power reactors. The UO2 is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity. 

Oxide fuels have demonstrated very satisfactory high-temperature, dimensional, and radiation stability and chemical compatibility with cladding metals and coolant in light-water reactor service. Under the much more severe conditions in a fast reactor, however, even inert UO2 begins to respond to its environment in a manner that is often detrimental to fuel performance. Uranium dioxide is almost exclusively used in light-water-moderated reactors (LWR). Mixed oxides of uranium and plutonium are used in liquid-metal fast breeder reactors (LMFBR). 

Uranium dioxide, UO2, is the compound of choice in many nuclear reactors despite its relatively poor heat conduction properties. This is due to its chemical stability, its high melting point, and the ease of production of well-characterized morphological and physical properties. The complete characterization is described in Chapter 2. Uranium metal and particularly uranium alloys like U-Al, U-Zr, U-Si, and U-Mo are also used as fuel. Their heat conduction is superior to that of uranium oxide but the metal and alloys are less stable chemically. 

Conditions must be carefully controlled to prevent sintering of the oxide particles during reaction in order to keep a high specific surface area of the oxide needed in the fluorination reaction. Nevertheless, in certain cases, if the uranium dioxide is to be used directly as nuclear fuel in heavy-water nuclear reactors (i.e., CANDU), sintering can be allowed to produce a denser ceramic fuel. 

Uranium Oxides. The important oxides of uranium are UO2, U3O8 and UO3. The dioxide (m.p. 2880 C theoretical density 10.%) is used as n nuclear fuel element. Uranium oxide has been used to produce red and yellow glazes and ceramic colours. 

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