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B How energy is released during nuclear fission?

  1. Mar 11, 2018 #1
    In a simple nuclear fission reaction an uranium atom breaks into krypton and barium releasing around 200MeV.
    Binding energy per nucleon (BE/A) of uranium = 7.6 MeV
    And binding energy per nucleon of krypton an barium is just larger than that of uranium.
    So in my view, around 200 MeV is used to increase the binding energy to bind the protons and neutrons in barium and krypton. ( I may be wrong ) But, from where the energy is released ( i.e energy from mass defect = 0.2253*951 = 208 MeV and around this amount of energy is used to increase the binding energy of barium and krypton)
    Is there some thing else happening or I am wrong about binding energy?
  2. jcsd
  3. Mar 11, 2018 #2
    have a look at the following...

    actually 85% of the energy coming out is from kinetic energy of fission fragments...and these fragments decay to more stable products, other radiations also play a role..the delayed neutrons play a role in sustaining the chain reaction.
  4. Mar 11, 2018 #3

    the excess masses of 236 U, 95 Kr and 141Ba are respectively -42.52 MeV, -58.34 MeV and -79.97 MeV ... totals to about 180 mev

    moreover taking the initial unit radius R0 = 1.48 fm and R1 and R2 the fragment radius at the point of contact

    the maximum of the curve of fragment separation is found to be at 201 MeV.

    so a neck formation takes place if it gets dipped by say 20 mev the threshold of fragmentation is reached.a detail analysis is presented in various text books or

    in fission energy

    the KE of fission fragments =165 MeV + other products neutrons , beta and gamma radiations, neutrinos add to about 35 Mev

    The Atomic Nucleus by J. M. Reid, Penguin Library of Physical Sciences,London, 1972
  5. Mar 11, 2018 #4


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    Staff: Mentor

    Remember, although we usually give binding energy as a positive number, it is actually negative in its effect on the mass of the system. So if the total binding energy of barium and krypton is larger than the binding energy of uranium, the sum of the masses of the final nuclei (barium + krypton) is smaller than the mass of the initial nucleus (uranium). This decrease in mass goes into the kinetic energies of the decay products.
  6. Mar 12, 2018 #5
    Thank You everyone i understood.
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