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Fusion vs. fission

  1. Dec 29, 2005 #1
    [SOLVED] fusion vs. fission

    I was trying to describe to my buddy the difference between fusion and fission and my explanation got a jumbled up, which made me realize how I myself dont really understand the difference. My understanding is...

    fission - two or more particles combines, splitting the nucleus which gets released as energy

    fusion - two or more particles are combined, creating a another elment as well as heat energy released

    am I close? why do we have fission reactors instead of fusion ones? Which one is used in the stars or atomics bombs?
  2. jcsd
  3. Dec 29, 2005 #2


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    Staff: Mentor

    Most commonly in fission, a neutron is absorbed in a fissile nucleus (U-233, U-235 or Pu-239 are most common) and the nucleus splits into two nuclides and two, three and sometimes rarely 4 neutrons. If at least one neutron survives on average from each fission, and that neutron is absorbed by another fissile nucleus, then the process is critical. Fission can be induced by bombardment of high energy particles or antimatter, but that is usually not practical for energy production.

    In fusion, the nuclei of light elements, e.g. D (deuterium, H2) and/or T (tritium, H3), combine to form new nuclei, one of which is heavier and the binding energy is released.

    See this for examples - http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2

    Stars use fusion as an energy production process.

    Atomic bombs use the fission process (Pu-239), but thermonuclear weapons use a fission trigger to ignite a fusion process.

    The commercial nuclear power industry uses fission reactors and have done so since the late 1950's/early 1960's. Now it is a muture technology. Fusion has yet to be perfected to the point where it is a commercially viable energy/power source.
    Last edited: Dec 29, 2005
  4. Dec 29, 2005 #3
    You have the basic idea correct. Fission reactors are primarily used instead of fusion reactors because the temperatures required for fusion to occur are extremely high. Stars use fusion, and atomic bombs use fission. (Hydrogen bombs use fission to trigger fusion.)
  5. Dec 29, 2005 #4


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    Saying "two or more particles combine" is confusing. Fission actually consists of a nucleus splitting into two smaller nuclei, total mass of the two being less than the mass of the original nucleus. That missing mass is where the energy comes from. When you say "two ofr more particles combining, you may be thinking of the neutron typically used to initiate the reaction. That does not "combine" with anything- it comes out again. In fact, you may get out a number of neutrons, which can start a chain reaction.
    Yes, that's pretty good. Typically it is two hydrogen atoms combining to form Helium- now the mass of the Helium is less than the mass of the two hydrogen nucleil.
    "Atomic" bombs, by definition, use fission. Hydrogen bombs use fusion. We use fission reactors because, as yet, no one knows how to contain a fusion reaction. Stars consist mostly of hydrogen so while there are, in fact, a number of different reactions going on, the heat and light is primarily due to fusion reactions.
  6. Dec 29, 2005 #5
    Fission vs Fusion
    Fission splits a massive element into fragments, releasing energy in the process (see attatchment).Fusion joins two light elements, forming a more massive element, and releasing energy in the process.

    Binding Energy Curve

    The reason they both release energy can be understood by examining a curve called the binding energy per nucleon curve.

    In fission, an element with a very large number of nucleons (such as Uranium) is split, forming two fragments which each have fewer nucleons (the total number of nucleons is always constant). These fragments are nearer the maximum of the curve, so the total binding energy increases. In fusion, very light atoms (before the maximum of the curve) are fused into a more massive atom, neare the maximum again.

    Check out Astronuc's link.

    Attached Files:

    Last edited: Dec 29, 2005
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