Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nuclear Fusion and the H-Bomb

  1. Mar 7, 2014 #1
    This is a total newb question but please explain. If nuclear fusion is occuring during a hydrogen bomb explosion, then why is it said that more energy is needed catalyze fusion than will be produced during the reaction - H-bombs seem to blow that theory out of the water. (no pun intended) :tongue:
  2. jcsd
  3. Mar 7, 2014 #2
    I believe the statement refers to controlled reactions - ones that can be used to harvest the energy? Otherwise we would need know the source of the statement to understand your question.
  4. Mar 7, 2014 #3
    Yes so I guess that's my question. Does the method for starting a fusion reaction vary that much from a controlled reaction to a bomb? And if so, what are the differences?
  5. Mar 7, 2014 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    H-bombs are two-stage devices. A thermonuclear bomb (H-bomb) relies on the radiation and heat produced by the detonation of a fission device (the primary) in order to initiate the fusion reactions produced by the secondary. The whole mechanism is consumed in the resulting blast, so that principle cannot be used to initiate a controlled fusion reaction.

    For a fusion reactor to work, the fuel must be induced to fuse without resort to using an A-bomb, which is a very difficult proposition. It takes a lot of energy (electric power) to create the high temperatures (on the order of millions of degrees C) necessary to cause fusion of hydrogen. There are significant engineering problems associated with keeping the hot fuel contained for extended periods during which a reactor would be expected to operate.
  6. Mar 7, 2014 #5
    For fusion to occur, you need the right isotopes very hot and preferably dense. In a tomahawk fusion reactor this is done by external heating of the fuel, in the form of a plasma caught in an electromagnetic field. Heating it, keeping it hot and producing the electromagnetic field all cost electrical energy. It turns out right now, the useful energy we get out of these systems is less than the useful energy we put in (although you do actually produce a small amount of excess heat).

    Hydrogen bombs work differently. They use an fission bomb (aka atom bomb) to get the fusion fuel hot enough. They also don't worry about trying to contain it. Hydrogen bombs can release very large amounts of energy but it is not controlled.
  7. Mar 7, 2014 #6
    Ok, that makes sense. But what about fission reactors? We are able to create those and contain them. Why then can't we use a small fission reactor as a catalyst for a larger fusion reactor? Maybe these are dumb questions, but I'm not a nuclear physicist and very curious about the technology. Thanks for the help!
  8. Mar 7, 2014 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

  9. Mar 7, 2014 #8
    Fussion reactors need high temperatures so that the atoms can collide hard enough that they can overcome the electric fields from the protons in the nucleus. There are some other ways of doing this, but none of them are currently considered as scalable to commercial power as tomahawks. "Cold fusion" would be making this happen without high temperatures.

    Fission reactors don't need to get hot to work. Fission is initiated when some heavy isotopes like U235 absorb a neutron. This makes it unstable and it splits into two pieces and gives off energy.

    An interesting thing about nuclear fuel is that iron has the highest binding energy of all isotopes. This is a fancy way of saying if we could convert any isotope to iron it would give off energy. So things that are lighter than iron will give off energy if they combine to make something heavy (getting closer to iron), and things that are very heavy like Uranium will give off energy if the split apart to get closer to iron. That is how both fission and fusion can release energy. Fission of a light element like helium would not give off energy (it would absorb energy) while fusion of two uranium atoms would also absorb energy (and would result in an unstable product which would decay).
  10. Mar 8, 2014 #9
    I have seen some designs for a hybrid DT fusion and thorium fission reactor at the University of Michigan, so your idea of a hybrid fission fusion reactor is not out of the question. The design is by Prof Emeritus Terry Kammash, but is mostly meant for propulsion.
  11. Mar 9, 2014 #10


    User Avatar

    Staff: Mentor

    Controlled fission and fusion systems operate under different (and more constrained) conditions than uncontrolled (explosive) fission or fusion systems, which is analogous to difference between deflagration/combustion/burning vs explosion.

    Uncontrolled fissile systems become prompt supercritical, and generated energy in microseconds, as opposed to controlled systems which operate normally in a critical state to maintain a constant/steady release of thermal energy. Typically, the material used in the process is highly enriched (fissile isotope) in the metal form.

    In modern commercial fission reactors, about 5% or so of the initial uranium is consumed over a period of three to four years (depending on energy density). A typical fissile reaction rate is on the order of 1019 fissions/cm3-s

    Similar in controlled fusion, the energy/reaction rate is constrained, and controlled fusion occurs at lower temperatures than are realized in thermonuclear weapons. The temperature (and energy density) is limited because the magnetic field strength and strength of structural materials are limited. Limits on magnetic field strength impose limits on the magnetic pressure that we can achieve in reactors.

    Hybrid fusion/fission devices exploit the neutrons occuring from fission, but that then involves producing fission products, which require disposal as spent fuel or high level waste.
    Last edited: Mar 9, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Nuclear Fusion and the H-Bomb
  1. How to make an H-Bomb (Replies: 18)

  2. Clean Fusion Bombs? (Replies: 57)

  3. Nuclear Fusion idea (Replies: 11)

  4. Nuclear fusion (Replies: 5)