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H vs. A; bombs

  1. Jun 10, 2008 #1
    I am an afficionado of all weaponry, and there is this question I have which has bugged me for a while. What, exactly, makes a Hydrogen bomb different from an atomic bomb? I realize that the explosive is Hydrogen as opposed to Uranium or Plutonium, but why is it more powerful?
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  3. Jun 10, 2008 #2


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    I think it is the amount of fuel: E = mc^2, in a hydrogen bomb you use fusion and hydrogen has no critical mass, hence you can have more mass in a safe state before you ignite the bomb. Also fusion relases more energy per unit mass than fission:


    the slope is greater when going from left to right (fusion) than going from right to left (fission)

    I hope this gave you the basic explanation.
    Last edited by a moderator: Apr 23, 2017
  4. Jun 10, 2008 #3


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    The binding energy per nucleon is higher for fusion than for fission. In fission, the U-235 or Pu-239 nuclei absorb a neutron becoming excited U-236 and Pu-240 respectively, which then fission releasing about 180 MeV promply (with about 20-25 MeV coming from delayed neutron, gamma and beta-decay in fission products), so fission releases less the 1 MeV/nucleon. The prompt part is key, because a fission device has on the order of 10 microseconds to fission as much as possible before the supercritical mass dissipates.

    Fusion (thermonuclear) weapons are actually initiated (triggered) by fission devices. The extreme heat of the fission detonation is used to heat the fusile material (e.g. d+t, or li+d) very rapidly. The d+t fusion reaction releases 14.6 MeV or about 3.5 MeV/nucleon.

    Another resource for understanding binding energy is -
    Last edited: Jun 10, 2008
  5. Jun 10, 2008 #4
    Well, thanks for clearing that up for me! Always great to learn something new, even at 14 :)
  6. Jun 10, 2008 #5


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    Another 'advantage' of fussion devices is that there is no real liitto how largeyou can make them.
    If you make a fission device too small you don't get a criticality and no bang. But if you try and put more fissile material together you get a pre-detonation where the inner part reaches critically and goes bang - blowing away the outer material before it has ha time to react.
    In a fusion device, the high temperature allows more hydrogen to fuse, which generates more energy, which fuses more material and so on....
  7. Jun 15, 2008 #6


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    Surprisingly, a fusion reaction releases less energy than a fission one -- about 20 MeV for H2+H3 -> He4 + n, versus 200 MeV for fission. You can have many more fusion reactions per gram of material, though, and as pointed out there is no limit to the size of a fusion bomb. The complete fission of 1 gram of U235 or Pu239 will produce about 70 GJ of energy, while the complete fusion of 1 g of a deuterium/tritium mix will produce 300 GJ.
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