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    * Uranium-235 *

    اورانیوم-۲۳۵ ،

    (Wikipedia) - Uranium-235 "U-235" redirects here. For the World War II submarine, see German submarine U-235. Uranium-235 General Nuclide data Decay mode Decay energy

    Uranium metal highly enriched in uranium-235

    Full table
    Name, symbol Uranium-235,235U
    Neutrons 143
    Protons 92
    Natural abundance 0.72%
    Half-life 703,800,000 years
    Parent isotopes 235Pa 235Np 239Pu
    Decay products 231Th
    Isotope mass 235.0439299 u
    Spin 7/2−
    Excess energy 40914.062 ± 1.970 keV
    Binding energy 1783870.285 ± 1.996 keV
    Alpha 4.679 MeV

    Uranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.

    Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its (fission) nuclear cross section for slow thermal neutrons is about 504.81 barns. For fast neutrons it is on the order of 1 barn. Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.


    FissionNuclear fission seen with a uranium-235 nucleus

    The fission of one atom of U-235 generates 202.5 MeV = 3.24 × 10−11 J, which translates to 19.54 TJ/mol, or 83.14 TJ/kg. When 235 92U nuclides are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown visually in the image to the left):

    1 0n + 235 92U → 141 56Ba + 92 36Kr + 3 1 0n

    Heavy water reactors, and some graphite moderated reactors can use unenriched uranium, but light water reactors must use low enriched uranium because of light water''s neutron absorption. Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235. Highly enriched uranium, which contains an even greater proportion of U-235, is sometimes used in nuclear weapon design.

    If at least one neutron from U-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments which are highly radioactive and produce further energy by their radioactive decay. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.

    Nuclear weapons

    The Little Boy gun type atomic bomb dropped on Hiroshima on August 6, 1945 was made of highly enriched uranium with a large tamper. The nominal spherical critical mass for an untampered 235U nuclear weapon is 56 kilograms (123 lb), a sphere 17.32 cm (6.8") in diameter. The required material must be 85% or more of 235U and is known as weapons grade uranium, though for a crude, inefficient weapon 20% is sufficient (called weapon(s)-usable). Even lower enrichment can be used, but then the required critical mass rapidly increases. Use of a large tamper, implosion geometries, trigger tubes, polonium triggers, tritium enhancement, and neutron reflectors can enable a more compact, economical weapon using one-fourth or less of the nominal critical mass, though this would likely only be possible in a country that already had extensive experience in engineering nuclear weapons. Most modern nuclear weapon designs use plutonium as the fissile component of the primary stage, however HEU is often used in the secondary stage.

    Source Average energy released
    Instantaneously released energy
    Kinetic energy of fission fragments 169.1
    Kinetic energy of prompt neutrons     4.8
    Energy carried by prompt γ-rays     7.0
    Energy from decaying fission products
    Energy of β−-particles     6.5
    Energy of delayed γ-rays     6.3
    Energy released when those prompt neutrons which don''t (re)produce fission are captured     8.8
    Total energy converted into heat in an operating thermal nuclear reactor 202.5
    Energy of anti-neutrinos     8.8
    Sum 211.3

    Tags:German, Science, U-235, Uranium-235, Wikipedia, World War II

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