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

Nuclear bomb

  1. Jul 19, 2010 #1
    Sorry for silly question but I can't find the answer anywhere. Is any kind of
    moderator used in nuclear bomb (or the mass of the uranium is so big
    that neutrons speed need not to be reduced). Thanks.
  2. jcsd
  3. Jul 19, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Bombs don't use a moderator because they contain mostly highly enriched fissile material.
    Although fast neutrons have a lower probability of fissioning an atom (which is why you use a moderator in a reactor) there are so many fissionable atoms around them in a bomb that they have a much higher chance of causing another fission event.

    I think there were attempts to use a moderator for some of the first thermonuclear devices, you need the initial reaction to last a bit longer and have more neutrons flying around to heat the fusion reaction. But I think most modern designs use a reflector instead.
  4. Jul 20, 2010 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    Fissile nuclear weapons are designed for 'prompt supercritical' conditions, i.e., the fissions are induced by fission neutrons with energies in the MeV range. The detonation reaction occurs over microseconds, so no time to be moderated. The vast majority of fissions must occur before the supercritical mass disperses. In addition to very high enrichment, an implosion compresses the material, thus increasing the density somewhat.
  5. Jul 20, 2010 #4


    User Avatar
    Science Advisor

    Implosion is necessary for Pu239 bomb, but not for U235 bomb.
  6. Jul 21, 2010 #5
    Some bombs use beryllium or tungsten carbide neutron reflectors.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook