Isotope enrichment process

Ceramic membranes were first developed in the 1940s for uranium isotope enrichment processes. Important progress has been made since that time, mainly due to the improved knowledge of the physicochemical properties of the membrane precursors. Most CMR studies concern alumina membranes other oxides such as silica, titania, or zirconia are much less frequently mentioned. 

Many passes are made, each increasing the fraction of UFg until a mixture is obtained that contains enough UFg. This isotope-enrichment process was developed during the latter years of World War II and produced enough for two of the world s first three atomic bombs. The principle is still used to prepare nuclear fuel for power plants. 

The present process still depends on the production of UF4 as a pure intermediate which may be reduced to metal for fueling the Magnox reactors or further fluorinated with fluorine gas to produce UFg, the essential feed material for all of the uranium isotopic-enriching processes. 

Because of its use in the Manhattan Project, the details of the electromagnetic isotope enrichment process were highly classified. After the discontinuation of its use for enrichment of uranium, much of the related technology was declassified and made available to the rest of the world through conferences and technical publications. Many countries developed their own electromagnetic isotope enrichment capability, but much smaller in scale. While the individual separators were similar in size and design, the number of separators, and thus the total production capability, was much smaller. Russia pursued a course similar... 

Natural (NU or Unat), depleted (DU), low-enriched (LEU), and high-enriched (HEU) uranium the content of the only natural fissile isotope, U—is an important feature of uranium applications and value. In natural uranium, the content of this isotope is 0.720 atom % or 0.711 wt% (Table 1.2). LEU is defined as U content between 0.720% and just below 20%, while HEU encompasses uranium with U content above 20%. The 20% borderline between LEU and HEU is artificial and was based on the assumption that nuclear weapons with 20% or less U would not be efficient. The waste, or tails, of the isotope enrichment process contains less U than in natural uranium and is defined as depleted uranium (DU). The U-235 content in DU is usually in the range of 0.2%-0.4%. DU is used mainly in armor piecing ammunition, in reactive armor of tanks, in radiation shielding, and is also used as ballast weights in aircraft. In addition, many of the commercially available fine chemicals of uranium compounds are based on the tails of uranium-enrichment facilities and usually labeled as not of natural isotope composition. 

Uranium hexafluoride (UF ), also called hex, is probably the best known and most widely investigated compound of uranium mainly because it is the only uranium compound with significant vapor pressure at ambient temperatures and therefore an essential raw material for most commercial isotope enrichment processes. UFg is a white monoclinic crystalline solid that sublimes directly to a gas (reaches atmospheric pressure at 56.5°C), but when heated in a closed vessel will melt at 64.05°C, which is the triple point where the solid, liquid, and gas phases coexist, as shown in Figure 1.8. This is probably one of the most weU-recognized phase diagrams in the chemical literature. 

In natural uranium ores, the fraction of the atoms of the fissile isotope is about 0.72%. For many commercial applications, like production of fuel for light water reactors or several types of research reactors and other nuclear functions, its fraction must be increased, that is, isotope enrichment is carried ont. The main isotope separation methods, or isotope enrichment processes, ntilize the small differences in between the mass of U-235 and U-238. The two major commercial methods that have supplied most of the enriched uranium to date, gaseous diffusion and gas centrifuges, use the only gaseous compound of nraninm, nranium hexafluoride (UFg), as the feed material. Both methods utilize the difference between the mass of UFg (349 Da) and UFg (352 Da) where the mass ratio difference that is 0.86%. The product and tails of the enrichment process are also with the same chemical form, but the isotope composition of the material is altered in the enrichment process. Schematic diagrams of the principle of operation of these methods can be found on the web and in many textbooks, so will not be shown here. 

The production of UFg at the UCF and, more importantly, the isotope enrichment process, are inevitably accompanied by some release of gaseous UFg. Once UFg is released into the atmosphere, it will react with moisture to form aerosols of uranyl fluoride and HF (Equation 5.1) ... 

The isotope enrichment process does not only lead to an increase in the concentration in the fuel, but also in a more than proportional increase in the concentration, as can be seen from Table 3.1., where the mass concentrations of the actinide isotopes in different types of fresh nuclear fuel are compared. As a... 

Any separation Victor describing such an isotope enrichment process has to take into account the vapor-liquid equilibrium for the vapor species HCN. Consider now the isotope exchange reaction (5.2.49) equilibrium along with the vapor-liquid equilibrium of HC N and HC N. There are a total of six mole fractions, Xy, to deal with ... 

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). 

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). 

G. F. Mailing and E. Von H.a]le,Merocfnamic Isotope Separation Processes for Cranium Enrichment Process Requirement, paper presented at the Symposium on New Advances ia Isotope Separation, Div. of Nuclear Chemistry and Technology, American Chemical Society, San Francisco, Calif., Aug. 1976 CCC-ND Report K/OM-2872, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Term., Oct. 7, 1976. 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

Practicable isotopic enrichment has the following prerequisites adequately short time for the enrichment process, acceptable asymptotic enrichment factor, and adequate accuracy for the estimation of the enrichment factor. (When total activity, rather than specific activity, is limiting, one must also pay attention to losses during enrichment.) For the argon and carbon enrichments referred to above, enrichment factors of about 100 and 500 were obtained within a week and a few hours, respectively and enrichment factors were deduced from direct observations of adjacent, stable isotopes. The 14C enrichment process provided extra dividends for AMS measurement the sample was implanted in an ideal form for the accelerator ion source, and it was spatially localized (depth) which gave added signal-to-noise enhancement. 

In contrast, Meckenstock et al. [280] reported larger isotopic enrichments in residual toluene, 3-6%o and up to 10%o during anaerobic and aerobic biodegradation experiments, respectively. These results indicated that isotopic fractionation effects may be different for different compounds, terminal electron-accepting processes (TEAP), degradative metabolic pathways, or microbial populations. 

Figure 3. Lithium isotope data for a range of commercially-available synthetic concentration standards (Qi et al. 1997a). The inset expands the c. 60%o range of reported natural samples. Although most anthropogenically-processed Li retains a broadly terrestrial value, nearly 20% of the samples examined show enormous isotopic enrichment in the heavy isotope.

Regardless of the ultimate sources of these compositions, these results clearly show that strongly isotopically fractionated Li from crustal sources plays a role in the mantle. Processes active in subduction zones appear to be cardinal in the control of the Li isotopic composition of different parts of the mantle. The results to date imply that both isotopically enriched (8 Li > MORE) and depleted (5T i < MORE) material are available for deep subduction, and that areas of the continental lithosphere may retain these records on long time scales. 

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