Uranium-236 enrichment reductant

The nuclear area is one that has been heavily dependent upon isotope ratio mass spectrometry performed by thermal ionization. Applications in this area are among the major reasons for the continued push to analyze smaller and smaller samples. There are two primary reasons for this (1) maximum practicable reduction of the hazards associated with radioactivity and (2) presence of often only a very small amount of the target element available. Areas addressed include evaluation of uranium enrichment processes [86], isotopic analysis of transuranium elements (all elements through einsteinium have been analyzed) [87], and environmental monitoring for release of uranium and other actinides [88,89]. This last area has received renewed emphasis in the wake of the Gulf War [90]. 

National Research Council (U.S.). Committee on Decontamination and Decommissioning of Uranium Enrichment Facilities Affordable Cleanup Opportunities for Cost Reduction in the Decontamination and Decommissioning of the Nation s Uranium Enrichment Facilities, National Academy Press, Washington, D.C., 1996. 

Because plutonium recycle makes possible the generation of 3074 MWe of electricity with only 2074 MWe nquiring enriched uranium, the reduction in separative work demand nude possible by plutonium recycle is (100X3074 — 2074)/3074 = 32.5 percent, and the specific separative work demand is (106,974X2.074)/3074 = 72.1 SWU/(MWe-year). This is to be compared with 106.974 SWU/(MWe year) without plutonium recycle. 

A continuous ion-exchange isotope separation process for uranium enrichment has been developed in Japan. Few details of this process have been disclosed. However, it is known that "reflux" is obtained by oxidation and reduction of U " " and U02 . A demonstration plant with a capacity of 2 kSWU/y is in operation at Hyuga. 

Mineralization style in volcanogenic rocks is variable. Where porosity and permeability changes are rapid, reduction in temperature and pressure may be sufficient to reduce hexa-valent uranium and form pitchblende. Where porosity and permeability changes are less abrupt, cooling of the silica-rich uranium-enriched fluids may produce uranosilicates. 

Once enriched, the UFg needs to be reduced to either uranium metal or UO2 to be formed into fuel pins. A variety of methods can be used to accomphsh the conversion to the oxide however, the predominately used technique involves reduction of the UFe to U metal fully, using Ca at high temperatures, followed by burning in oxygen. Once formed, the UO2 is pressed into pellets, which are then fed into fuel rods. 

Uranium metal was used as fuel in early types of reactors. UFe, sublimating at 56 °C, is used to separate the isotopes U and U. After isotope separation, UO2 may be obtained from UFg by hydrolytic decomposition, precipitation of U as ADU and heating, and uranium metal may be produced by reduction with hydrogen to UF4 and further reduction with Ca. Handling of enriched uranium compounds requires small-scale operations with amounts of the order of 1 to 10 kg, depending on the conditions, to exclude criticality. 

Holmes D. E., Finneran K. T., O Neil R. A., and Lovley D. R. (2002) Enrichment of members of the family Geobacter-aceae associated with stimulation of dissimilatory metal reduction in uranium-contaminated aquifer sediments. Appl. Environ. Microbiol. 68, 2300-2306. 

In the reduction stripping process uranium(IV) in the raw wet process acid is oxidized to uranium(Vl) by treatment with sodium chlorate, hydrogen peroxide or air at 60 to 70°C, the uranium(VI) formed being extracted with trioctylphosphine oxide/di-(2-ethylhexyl)phosphate in kerosene and the resulting solution finally reductively stripped repeatedly with aqueous iron(II) solutions. This results in an enrichment by a factor of 40. After oxidation of the stripped solution with sodium chlorate or ambient oxygen and renewed extraction of the uranium(VI) formed with trioctylphosphine oxide/di-(2-ethylhexyl)phosphate, the phosphoric acid is removed from the organic phase by washing. The uranium(Vl) is then stripped with ammonium carbonate and precipitated as the ammonium diuranyl-tricarbonato-complex. This is filtered off, washed and calcined. 

In the Purex process, plutonium and uranium are coextracted into an organic phase and partitioned by reducing plutonium(IV) to the aqueous-favoring plutonium(III). This has been achieved chemically by use of a suitable reductant such as ferrous sulfamate ( 1) or uranium(IV). (2, 3, 4, 5) The use of ferrous sulfamate results in accelerated corrosion of the stainless steel, due to the presence of ferric ions and sulfuric acid, and in an increase in the volume of wastes. The use of natural uranium(IV) can cause dilution of the 235U in slightly enriched uranium, thus lowering the value of the recovered uranium. 

Enriched or depleted uranium is usually produced in the form of UF, but is used as metallic uranium or UO2. This requires conversion of UF to UF4 or UO2. UFg is converted to UF4 by vapor-phase reduction with hydrogen. Because the heat of reaction is small, the mixture must be heated. In small reactors used for converting highly enriched uranium, heat is provided internally by reacting fluorine with hydrogen. 

It is on occasion necessary to prepare uranium tetrafluoride from uranium hexafluoride, particularly when dealing with material highly enriched in U285. The conversion of uranium hexafluoride to the tetrafluoride has has therefore attracted considerable study, and the present situation is summarized by Smiley and Brater 80). Uranium hexafluoride can be reduced with hydrogen. The thermodynamic equilibrium, even at room temperature, is in favor of the formation of uranium tetrafluoride, but the reaction requires a high energy of activation to inaugurate reduction. The... 

Metallic uranium is usually produced by conversion of UFg to UF followed by metallothermic reduction of UF4 by magnesium or calcium metal however, several other methods exist. Metallic fuel is encased in a canning (cladding) of aluminum, magnesium, or their alloys. Fuel for high flux research reactors based on highly enriched uranium (> 10% U) is often made in the form of uranium metal alloys (or con jounds like USi2) canned in aluminum to improve mechanical and thermal stability. 

It has been demonstrated that a 12% reduction in the mass of all array units in a reflected cubic array of highly enriched uranium will cancel the increase in keff produced by optimum-density water-mist moderation. For nearly critical cubic arrays, 12% in array units mass can be converted to 4% in keff. The arrays presented with unchanging reflector (no water-mist walls) all had keff Increases of 4% or less when optimumly moderated. 

At one of toe larger LWR fuel fabrication facilities, very large containers of low enriched uranium contam inated scrap are assayed using gamma rays (from Pa, a daughter of with large Nal detectors, prior to volume reduction by incineration. These assays provide a measure of the total uranium input into the incinerator. Thus incinerator cleanout is required only when the uranium content reaches toe level specified by criticality procedures. Thid results in fewer incinerator shutdowns for cleanout. 

The inverse of uranium utilization is uranium consumption. Relative to a PWR, natural uranium requirements in an HWR are 30% lower with natural uranium fuel. Enrichments to 0.9% SEU and 1.2% SEU would increase the fuel bumup by a factor of 2 and 3, respectively, relative to natural uranium fuel. This amounts to 45% lower uranium consumption with 0.9% SEU. The reduction in mined uranium requirements also has environmental benefits at the front end of the cycle, which will become even more important in the coming decades as cheaper, higher-grade uranium ore resources are depleted, requiring the mining of greater volumes of lower-grade ores. 

Additionally, the health hazards posed by UFg, particularly radiation safety concerns, also depend on the level on enrichment (see Chapter 4). Uranium-containing deposits (called heels) may form in cylinders that are used for storage of UFg either through hydrolysis with moisture traces to produce UO2F2 or through reduction of UFg on the wall surfaces or with impurities. Accumulation of the heels gives rise to safety concerns and operational problems. 

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