Isotopic Content of Uranium

Uranium isotope separation plants may be fed either with natural uranium, which contains only the isotopes and in nearly fixed proportions or uranium discharged from a 

Until recently it has been assumed that natural uranium from all sources had exactly the same content of 1, and U. As lately as 1977, U.S. Energy Research and 

Development Administration (ERDA) used 0.711 w/o (weight percent) as the U content of all natural uranium feed supplied to U.S. ERDA plants for enrichment. However, accurate measurement of the U/ U ratios of uranium from various minerals and various locations has shown significant variations. Cowan and Adler [C9] have summarized measurements of the 

The difference between sandstone-type minerals and high-temperature minerals is considered to be significant. It is attributed probably to isotopic fractionation that occurred when uranium initially deposited at high temperatures from magmas was dissolved by water at lower temperature and reprecipitated in sandstones. The difference between non-U.S. and U.S. samples is explained in the same way, as most non-U.S. samples were of magmatic origin and most U.S. samples were of the sandstone type. 

Because of the possibility of natural depletion of U and because of the availability of tails from isotope separation plants that might become mixed with natural uranium, it is important that natural uranium feed for an isotope separation plant be analyzed for its U content. 

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Naudet, R. Renson, C. 1975. Resultats des analyses systematiques de teneurs isotopiques de I uranium. Results of systematio analyses of isotopic content of uranium. In Le phenomena d Oklo—The Oklo phenomenon Internatinal Atomic Energy Agency, Vienna, Austria (AUT). SCHAUBLE, E.A. 2007. Role of nuclear volume... 

Here is how SAL works Samples are received in a reception and storage room, then routed to the appropriate wet chemical analysis laboratory. There, they are analysed for uranium, thorium or plutonium content, and purified aliquots (portions of the sample) are prepared for the isotopic analysis of three elements. Isotopic analyses are performed routinely by mass spectrometry, and radiometric techniques are used for back-up. Emission spectrography serves to detect the presence of impurities which could interfere with the measurements and thus distort the results of the chemical and isotopic analysis of uranium, thorium and plutonium. Complex calculations and quality checks are performed on minicomputers, which are connected in a network to a central laboratory mini-computer. A central laboratory data system stores and provides analytical reports and enables the quality of the analyses and the status of the flow of samples through the laboratory at any time to be monitored. 

The present isotopic content of in natural uranium is 0.71 percent. 

The nominal isotopic content of natural uranium is 99.274 atom percent (hereafter a/o) U,... 

Natural radioactivity is formed particularly by long-lived isotopes, i.e. by those with half-lives of 10 to 10 years. These isotopes are usually widely scattered in the soil. The activity level depends particularly on contents of uranium, thorium, radium and potassium the radiation energy of these elements represents as much as 98% of the total energy of radiation of all the natural radioactive elements in the soil. 

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... 

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). 

Once a sample is collected, the isotopic composition of uranium must be determined as the content is one of the main factors that determine the price of the product. Several mass spectrometric techniques have been developed for direct isotope analysis of gaseous UFg and for indirect analysis (usually after hydrolysis) of liquid and gaseous UFg samples. The use of a thermal ionization mass spectrometer (TIMS), nowadays equipped with several detectors (i.e., multicollector TIMS), has been the method of choice for many years, but the sample must be hydrolyzed to liquid form (uranyl fluoride or uranyl nitrate solutions) and the uranium must be purified (usually not a problem for UFg samples), as mentioned, for example, by ASTM (C1413 2011). The method is used for hydrolyzed samples of UFg (UOjFj (uranyl fluoride)) or for... 

C1457 standard test method for determination of total hydrogen content of uranium oxide powders and pellets by carrier gas extraction Uranium isotopic analysis by mass spectrometry... 

Probably the most comprehensive published assay of DU used in armor pen-etrators was reported on the basis of analysis of an unfired CHARM-3 penetrator (Trueman et al. 2004). A sample from the penetrator was dissolved in 9 M HCl, spiked with U as a yield monitor, and the uranium was separated from impurities on an ion-exchange resin. The isotopic composition of uranium was determined by mass spectrometric techniques. Actinides ( - Am and Np) were determined in the uranium-free solution by gamma spectrometry and 239+24opy and Pu were measured by alpha spectrometry and their presence was confirmed by ICPMS. Technetium-99 was determined by ICPMS when rhenium was used as a carrier and interferences from iron were eliminated by precipitating with ammonia while ruthenium and molybdenum were removed by separation on a chromatographic resin. The content of these radioactive nuclides is summarized in Table 2.7. 

The rigorons specifications for nuclear grade materials that are used as nuclear fuel (mainly UO2 and U metal or alloys) or as feed material for enrichment facilities (primarily UFg) are described in great detail and require strict control. The focus of the analytical procedures is on impurities that affect the nuclear properties (mainly through absorption of neutrons), chemical properties (Uke corrosion resistance or those that may concentrate in the enrichment product), and physical and mechanical properties (like pellet strength, heat transfer). The isotopic composition of uranium plays an important role as the value of uranium strongly depends on the content. 

At the stage of spent fuel reprocessing, the accumulated plutonium is separated from uranium together with the accumulated minor actinides, which makes such plutonium ineffective for weapon devices. Also, the isotopic content of plutonium does not meet the requirements to weapons-grade plutonium ... 

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). 

During the initial stages of development, it was assumed that the isotopic composition of plutonium in the fresh fuel would correspond to reactor-grade plutonium extracted from the cooled spent fuel of a typical light water reactor (e.g., from a 900 MW(e) PWR fuel irradiated up to a bum-up of 33 MW-d/kg U, reprocessed after 10 years of cooling) and loaded in the RBEC-M reactor in two years. The isotopic composition of uranium corresponds to depleted uranium with a content of 0.1 weight %... 

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). 

Neutron-rich lanthanide isotopes occur in the fission of uranium or plutonium and ate separated during the reprocessing of nuclear fuel wastes (see Nuclearreactors). Lanthanide isotopes can be produced by neutron bombardment, by radioactive decay of neighboring atoms, and by nuclear reactions in accelerators where the rate earths ate bombarded with charged particles. The rare-earth content of solid samples can be determined by neutron... 

Exposure Levels in Humans. Although some data on the levels of americium in human tissues exposed to natural background levels (food, water, and air) are available, few measurements have been made on the americium content in human tissues. The principal source of information about occupationally exposed individuals is the U.S. Transuranium and Uranium Registries (USTUR) Tissue Program and database, established to document levels and distribution of uranium and transuranium isotopes in human tissues for occupationally exposed workers (USTUR 1999). Several major database files are available. 

The conclusions of Hurt s study of year-by-year oxygen isotope ratios in 72 years of S. gigantea are thus supportive of the conclusions of the CIAP study [49] that solar variations influence the abundances of many kinds of chemical species in the stratosphere, and therefore influence the.amount of solar energy they absorb and re-radiate to earth, and therefore influence the surface temperature of the earth and especially the surface temperatures of the oceans. It is the surface temperature of the oceans which produces the phenomena we have discussed the isotope ratio variations in rain and hence in tree rings, the isotope ratio variations in the Greenland ice cap, in the organic carbon and uranium concentrations in sea cores, and furthermore variations of the sea surface temperature produces variations in the carbon-14 to carbon-12 ratio fractionation at the sea air interface and hence in the carbon-14 content of atmospheric carbon dioxide and hence in the carbon-14 content of tree rings. 

Isotope One of several radionuclides of the same element (i.e., with the same number of protons in their nuclei) with different numbers of neutrons and different energy contents. A single element may have many isotopes. Uranium, for example, may appear naturally as 234U (142 neutrons), 235U (143 neutrons), or 238U (146 neutrons) however, each uranium isotope has 92 protons. 

Radioactive decay of the two main isotopes of uranium produces significant contents of the lead isotopes ° Pb and... 

But nature provides only 0.7% of the fissionable isotope U in natural uranium, the rest being 238u and although the predominant hydrogen isotope in water is a good moderator, it absorbs too many neutrons to allow a reaction to be maintained with the low content of natural uranium. However, the heavy hydrogen... 

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