Enriched uranium , isotope content

Uranium with isotopic abundances different from that of natural uranium is the primary signature for HEU production activities. In any separation technology some enriched uranium will inevitably be released to the environment. Environmental samples taken at or near an enrichment facility can contain some of the enriched material altering the uranium isotopic abundance. Analysis of samples of vegetation, water and soil for uranium isotopic content using a sensitive analytical technique, such as thermal ionization mass spectrometry is recommended as the primary technique for the detection of HEU production. 

Table 12.5 shows expected isotopic uranium contents which might be expected as product from cascade type (gaseous diffusion or gas centrifuge) enrichment facilities. While the highly enriched U is actually the target of these efforts one should also be able to verify the production of low enriched uranium. Columns 5 and 6 of Table 12.5 show the isotopic enrichments which would result from a 1000-1 mixture of namral U and low or high enriched U. Table 12.6 shows the ratios of the isotopes U and U relative to the in each mixture. The 0.6% isotopic shift in the ratio in the case of LEU-MIX should be detectable by today s technology. The 0.13% in ratio... 

It is recognized, of course, that other arrangements can be employed than those shown and the uranium may be in one of a variety of forms. For example, the uranium may vary in size and shape from small particles to larger lumps or bodies in any convenient shape, such as spheres, tubes, or rods. The uranium may be in the form of metal, or it may be In a compound such as UO2 or U3O11. Unless otherwise. specified reference to uranium bodies is Intended generically to mean bodies including the metal or its compounds. If desired, the natural uranium in such bodies may be enriched as to its isotope content, or... 

Natural uranium has an isotopic content of U-235 of only 0.72% (i.e., fraction = 0.0072). Present-day light water reactors are designed to use uranium with an enrichment of between 3% and 5% (see Table 24.5). 

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. 

Attempts at materials diversion from spent fuel would encounter all the challenges discussed above, plus performing the required activities in a high radiation field. The enrichment step could be eliminated by chemically separating out plutonium. However, the isotopic content of the plutonium in the spent fuel is not attractive for weapons use due to the neutronic characteristics of the GT-MHR LEU cycle. The quantity of fissile material (plutonium and uranium) per GT-MHR spent fuel element is low (50 times more volume of spent GT-MHR fuel elements would have to be diverted than spent light water reactor fuel elements to obtain the same quantity of plutonium-239). 

The only current industrial application of centrifugal separation of gases known to the author is the separation of uranium isotopes. Natural uranium is about 0.7% balance The uranium enrichment process increases the content to about 2 to 3% for electric power reactors and to about 90% for nuclear weapons, (There are several types of uranium enrichment processes [1], of which only the centrifuge process is discussed here.) All enrichment processes currently in use convert the uranium to uranium hexafluoride, UFg, which sublimes to a gas (as does solid carbon dioxide, dry ice ) at one atmosphere and 56.5°C. The resulting gas is 0.7 mol% (M = 349 g/mol) and... 

HIGHLY ENRICHED URANIUM (HEU). Uranium having a uranium-235 isotope content of 20 percent or greater is referred to as highly enriched. When the uranium-235 content reaches or exceeds 90 percent, then the material is regarded as weapons-grade HEU or, simply, weapons-grade uranium. See also LOW-ENRICHED URANIUM (LEU). 

LOW-ENRICHED URANIUM (LEU). Uranium that has been processed to increase the uranium-235 content (that is, has been enriched ) but still contains less than 20 percent of that isotope. LEU can sustain a chain reaction under certain conditions and, thus, is used as fuel in light-water reactors (LWR). Further processing is required, however, to make the material weapons-grade suitable for use as a nuclear explosive. See also HIGHLY ENRICHED URANIUM (HEU). 

The nuclear fuel used in almost all commercial reactors is based on uranium in the form of UO, either enriched so that the content has been increased to a few percent, or — less commonly — with the natural 0.7 % abimdance of the fissile isotope Some power reactors also use fuel containing depleted uranium ( 0.3% " U) in which plutomum is bred and/or Pu mixed with as a replacement for ("mixed oxide fuel"). Th, in which fissile is bred, has also been used in a few cases. 

Nuclear power plants use nuclear fission to generate energy. The core of a typical nuclear reactor consists of four principal components fuel elements, control rods, a moderator, and a primary coolant ( Figure 21.18). The fuel is a fissionable substance, such as uranium-235. The natural isotopic abundance of uranium-235 is only 0.7, too low to sustain a chain reaction in most reactors. Therefore, the content of the fuel must be enriched to 3-5% for use in a reactor. The fuel elements contain enriched uranimn in the form of UO2 pellets encased in zirconium or stainless steel tubes. 

The fuel cycle starts with the mining of the uranium. It continues with its chemical isolation, possibly an isotopic enrichment of (see O Chap. 51 in Vol. 5 on Isotope Separation ), the manufacture of the fuel elements, and their use in the reactor. If a final storage of spent fuel elements as such, as practiced in some countries, is not preferred, the cycle continues with a dissolution of the fuel elements. The remaining uranium and the newly formed plutonium are separated fi om the fission products. Plutonium can be reintroduced into the reactor in the form of Mixed OXide (MOX) fuel elements. Uranium has to pass through an enrichment plant in order to increase the content of from about 0.8% in the spent fuel to about 3%. The enriched fraction will be returned to new fuel elements. 

Isotope dilution analysis (IDA) involves mixing an aliquant of the sample with an aliquant of an artificially enriched isotope of the element to be analyzed. The content of the latter in the original sample is derived from the results of the isotopic analysis of the mixture compared to the isotopic compositions of the sample and of the enriched isotope. The term spiking is often used to refer to the mixing step. The aliquant of enriched isotope added to the sample is also commonly called a spike. IDA is used in particular for the determination of uranium, plutonium, or thorium in solutions of irradiated nuclear fuels. It has also been used for an independent measurement of the total volume or mass of solution in accountability tanks of chemical processing plants. 

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