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Why does FISSION release energy?

  1. Aug 25, 2009 #1
    I understand why Fusion releases energy..
    The product is "lighter" than the reactions and the missing mass is converted into energy (like gamma radiation). This energy can also be called binding energy as it acts to stabilize the atom

    Now, why would FISSION RELEASE energy?
    Wouldn't energy be required (in other words , shouldn't energy be put in) to split the atom in the first place?
  2. jcsd
  3. Aug 25, 2009 #2
    As a matter of fact, you need to "put" energy to split the atom, to set it above the treshold. This is done, in nuclear reactors, by "trhowing" a neutron on the uranium atom. Otherwise, you would have to wait many millions of years for it do disintegrate, with a very small probability of fissioning.

  4. Aug 25, 2009 #3
    Whether fission or fusion releases energy depends on the binding per nucleon, which depends (among other things) on the mass of the nucleus. This is summarized in the binding energy curve :
    See for instance Nuclear Binding Energy (hyperphysics)
  5. Aug 25, 2009 #4
    That's exactly why there is an upper limit that is energy favorable around the iron atom.
  6. Aug 25, 2009 #5
    How so?
  7. Aug 28, 2009 #6
    There are 3 ways that they release energy.

    By splitting the atoms into lighter products, the kinetic action of the split will generate heat. I believe what humanino said by binding per nucleon applies to this. The higher energy per binding, the more heat generated after collision.

    Second way is the residual heat that is produced from the decay of the materials long after the products have been depleted

    And lastly it is the gamma rays that are emitted that produce thermal energy.
  8. Aug 28, 2009 #7
    The semi-emperical mass curve is usually on a log scale. This linear scale really demonstrates how much more energy there is per nucleon when fusing H rather than fissioning something heavy.
  9. Aug 31, 2009 #8
    Said a bit more simply:

    How much binding energy neutrons & protons have is not constant. In fact, it is a function of the number of neutrons and protons in the nucleus. The curve that they are showing shows binding energy / (neutrons + protons).

    To get the energy out you want to move closer to the peak binding energy / (neutrons + protons) (which is the iron peak) this can be done from either side. For fusion, this means moving right along the curve. For fission, this means moving left.
  10. Aug 31, 2009 #9
    ahh... thanks!
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