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Why there is more energy released in fusion than fission?

  1. May 4, 2008 #1
    Why more energy is released in fusion than fission?
     
  2. jcsd
  3. May 4, 2008 #2

    Astronuc

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    That would be in terms of mass or per nucleon. The energy released has to do with the relative energy or binding energy per nucleon.

    Each fission reaction releases about 200 MeV of energy, but the initial mass is about 236 or 240 amu (after neutron capture). The fission reaction produces two new nuclei of masses about 143 and 90 amu and 2 or 3 neutrons. The two nuclei tie up a lot of energy, but are relatively stable with respect to additional fission (i.e. they will not fission but undergo other decay modes). But the main fact is that the lighter nuclei, although they have a greater binding energy per nucleon, that binding energy per nucleon is not significantly greater than the heavier fissile nuclei.

    In contrast, in fusion, the reacting nuclei (often the lightest of elements, e.g. H, He, Li, . . .) have very low binding energy per nucleon, but as we go from H to He to Li, the binding energy increases significantly. It is the large difference in BE per nucleon between say, d or t and alpha, which is responsible for the energy released as a result of fusion. Other fusion reactions are not so energetic, e.g. d + d -> p + t or n + He3.


    Please review this -
    http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2
     
  4. May 5, 2008 #3
    And why in fusion the nucleuses must be accelerated in order to have nuclear reaction?
     
  5. May 5, 2008 #4

    jtbell

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    The nuclei repel each other because of the electrostatic force between them, until they get close enough that the strong nuclear force becomes more important.
     
  6. May 5, 2008 #5
    Yes, sorry I forgot that. And are the neutrons from fission in preceding generation, in fission chain reaction (the 2-3 neutrons which are released) accelerated? In that case they won't be able to make another fission in the next generation.
     
  7. May 5, 2008 #6
    Are the neutrons released from fission, fast losing their energy? Do they become slow?
     
  8. May 5, 2008 #7

    Astronuc

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    Neutrons from fission are fast neutrons with E > 1 MeV.

    In light water reactors (LWRs), fast neutrons collide with protons in the water to slow down to thermal energies (~0.025 eV or so). Reactors using heavy water, D2O, use neutrons of slightly higher energy to induce fission.
     
  9. May 5, 2008 #8
    Hope u don't mind me tagging along your post.

    In fussion, atoms give out a chunk of mass as energy and use another chunk of mass to construct binding energy. In fission, it use the chunk of mass in binding energy and release as energy. My question is... in between, there must be an element that when fission or fussion doesn't give out any energy. I know, it's wack and probably no experiment done yet, but can we find it by calculation?
     
  10. May 7, 2008 #9
    Practically, you cannot get fusion, starting from Iron.
     
  11. May 7, 2008 #10

    malawi_glenn

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    What do you mean by "practically" ? You can still fuse iron and heavier against each other, but you dont gain energy in those processes - you'll loose energy instead. That is why stars starts to "die" when they reach to iron in their core (roughly speaking).
     
  12. May 7, 2008 #11
    Total newb here, so please point out my lack of understanding.
    Can elements heavier than iron actually fuse? I thought the way the heavier elements were formed was when a star was about to collapse and iron (or others) picked up stray protons or alphas and became the next element in the line.
    Ok, how far off am I?
     
  13. May 8, 2008 #12

    malawi_glenn

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    Well you CAN fuse higher than iron, the thing is that you cant gain energy anymore. You must add more energy than what is beeing released in the process of fusion. A star must in its core produce energy that can prevent the gravitational collapse of the star (this energy are photons that is produced in the fusion, and they will constitute radiation pressure).
    When you reach iron in the core of the star, in the fusion chain, the stars energy will be drained, and can't withstand the outer pressure of gravity, so it will collapse and explode.

    Well, you admit that iron pick up protons and alphas, that is also a fusion process.

    This are of course more delicate and detailed than what I just described, a good reference is "Nuclear physics of stars" by Iliadis, and "Cauldrons in cosmos" by wolf.
     
  14. May 8, 2008 #13
    Ah yes, silly me. When saying fuse, I was thinking of similar atoms(ie iron + iron), not just adding particles (which is, of course, also fusion). Is it possible to get heavier elements to fuse together, or would the energy required be too large?
     
  15. May 8, 2008 #14

    malawi_glenn

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    As far as I know U+U is a standard process in heavy ion collider physics.
     
  16. May 8, 2008 #15

    Astronuc

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    If one visits www.webelements.com, and selects the superheavy elements, there is a description of each element is made from heavy ion collisions.

    Seaborgium

    Darmstadtium

    There are a few endothermic fusion reactions:

    p + t -> 3He + n

    t + 6Li -> 7Li + p + n


    Basically the desirable fusion reactions are those which have a high Q value, or high mass defect or mass difference between reactants and products. It's the difference in binding energies of the reactants and products, which allows for an exothermic fusion reaction. Since Fe and Ni have the highest binding energy per nucleon, Fe or Ni fusion not produce useful energy. There would be too much lost from scattering collisions, which is a problem even for the most advantageous fusion reactions.

    Stellar nucleosythesis is based on fusion under conditions which cannot be recreated on earth in large reactors because of the enourmous pressures and energy densities involved. We just cannot construct sufficiently strong magnets or materials to enable controlled fusion process on the stellar scale.
     
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