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Really struggling with Fission

  1. Mar 18, 2012 #1
    Hi all

    I am struggling with the fission of U235 into Barium 144 and Krypton 89.

    U= 235.044u +(1.0087 mass of neutron)=236.053

    Ba=(143.923) + (Kr=88.917) +3(1.0087u)

    This leaves 0.186u missing on the products side.

    If this is converted to MeV I gain the value, 173MeV. This agrees with various sources on the difference in binding energy of the different isotopes.

    HOWEVER most sources then go onto say that in a single fission reaction more than 200 MeV is released per fission reaction. I can not reach this number at all. Even accounting for neutrino's making some of that energy inaccessible.

    My second problem is I can not really wrap my head around the concept this releasing energy...

    With fusion, the mass of the products and the binding energy required is less than that of the constituent particles, therefore the missing mass manifests itself as energy, easy.

    However even though the binding energy per nucleon is less for Uranium:

    Binding energy for an entire uranium atom 1783MeV,
    Krypton 766.909MeV
    Barium 1190MeV

    (I looked up those binding energies and they came out slightly differently to how I calculated them using (no of protonsx1.0073)+(no of neutrons) - mass of nuclues)

    The binding energy required to hold the products together is HIGHER, than the energy required to hold uranium together.

    Somebody please help explain this to me:(
  2. jcsd
  3. Mar 18, 2012 #2
    Binding energies are negative, so the higher binding energies of the products mean that energy is released.

    Binding energy is not something you have to put in to 'glue' the constituents together, it is actually a measure of the energy you would have to put in to drag them apart. The (strong) nucelar force is sufficient to overcome the electrostatic force with a little bit to spare; that difference is what constitutes the binding energy.
    Last edited: Mar 18, 2012
  4. Mar 19, 2012 #3

    So why the MeV different then? 173 produced to upwards of 200?
  5. Mar 20, 2012 #4
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