Fission of U-235 by Neutrons

FIGURE 1.7 The mass distribution of the fission products from MOX fuel. (From World Nuclear Association, http //www.world-nuclear.org/uploadedImages/org/info/Nuclear Fuel Cycle/distribution of fission products.png, accessed July 26, 2014. With permission.) 

Energy (keV) and Probability (%) of Emitted Alpha Particles and Gamma Rays of the Major Isotopes of Uranium 

Source Based on Firestone, R.B. et al., Table of Isotopes, book and CD-ROM, 8th edn., John Wiley Sons, Inc., New York, 1996. 

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 that is used to fuel light water nuclear power reactors is generally enriched to a level of 3%-5% and considered as LEU. Several reactors that are used for research, material testing, and production of isotopes for medical and industrial applications also use LEU fuel, sometimes with as much as 19.75% U. Other research reactors and nuclear-powered ships and submarines require a higher content of the fissile isotope in order to reduce the size of the reactor core. Weapon grade uranium typically contains around 90% U (HEU). Thus, the value and cost of uranium is strongly dependent on the fraction of U present. Some sources also refer to 

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