Uranium fuel fabrication

Releases from the nuclear fuel cycle may occur during reactor operations, nuclear fuel reprocessing, UFg conversion, uranium enrichment, uranium fuel fabrication, high-level waste solidification, high-level and low-level waste disposals. The enrichment process, with a significant aimual release through liquid discharges to the sea, is considered as the key contributor. So far, the total release of Tc to the... 

Uranium fuel fabrication covers the full spectnim of criticality safety technology. UFa is handled in ton quantities, sale by virtue of undermoderation UFa to ADU or... 

On the comparative economics of the system, we find that on the basis of present day prices for uranium, fuel fabrication and reprocessing, for the smaller size of reactor the slightly enriched system gives greater economy. On longer-term predictions the natural uranium system can compete. The differences are never very great. 

The operational costs of a plutonium-fuel fabrication facility are expected to be two to three times those currently encountered for the uranium-based fuel fabrication facilities, because of such considerations as personnel safety, physical security, and environmental restraints. Plutonium-fuel fabrication times are also expected to be increased by factors of 2 or 3 when compared to uranium-fuel fabrication times for similar reasons. 

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. 

The most important role of UO3 is in the production of UF4 [10049-14-6] and UF [7783-81-5], which are used in the isotopic enrichment of uranium for use in nuclear fuels (119—121). The trioxide also plays a part in the production of UO2 for fuel peUets (122). In addition to these important synthetic appHcations, microspheres of UO3 can themselves be used as nuclear fuel. Fabrication of UO3 microspheres has been accompHshed using sol-gel or internal gelation processes (19,123—125). FinaHy, UO3 is also a support for destmctive oxidation catalysts of organics (126,127). 

Enriched UF6 is processed into U02 powder at fuel fabrication facilities using one of several methods. In one process uranium hexafluoride is vaporized and then absorbed by water to produce uranyl fluoride, U02F2, solution. Ammonium hydroxide is added to this solution and ammonium diuranate is precipitated. Ammonium diuranate is dried, reduced, and milled to make uranium dioxide powder. The powder is pressed into fuel pellets for nuclear reactors. 

As discussed earlier, natural uranium is 0.72 atom % 235U, and the fuel used in light-water reactors is typically 3% 235U. This means the refined uranium must be enriched in the lighter 235 isotope prior to fuel fabrication. This can be done by a... 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

One kilogram of fuel provides 360 mWh of electrical energy (1.22 gBtu). To obtain 1 kilo of fuel requires 9 kilos of uranium. The cost of conversion, enrichment, and fuel fabrication results in a total cost of about 4,000/kg. Therefore, the fuel cost is 1.1c/kWh or 3.22/mBtu. 

Normalized Uranium Effluent Discharges from Uranium Mining, Milling, Conversion, Enrichment, and Fuel Fabrication... 

Although no studies were located that specifically tested immunological effects in humans following inhalation exposure to uranium, all epidemiologic studies of workers in uranium mines and fuel fabrication plants showed no increased incidence of death due to diseases of the immune system (Brown and Bloom 1987 Checkoway et al. 1988 Keane and Polednak 1983 Polednak and Frome 1981). 

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). 

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). 

Liquid discharges containing uranium resulting from uranium emichment and fuel fabrication are generally quite small (see Table 5-1). 

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