The concept is that a trace amount of material with an extremely high fast neutron absorption cross section can be alloyed with the fissile material, so that you can assemble a much larger mass of the fissile material via implosion before a chain reaction starts. Then once the reaction starts, the neutron poison burns away quickly, causing the device to rapidly become far more supercritical. Could this work to boost the efficiency of a nuclear weapon?

Are there any such isotopes with a significantly higher fast neutron absorption cross section than the fissile isotopes' fast neutron fission cross sections? I'm looking for at least 10 barns at 1MeV, and absorbing the neutron without releasing any other neutrons. I've looked and haven't been able to find any isotope with that property.

  • $\begingroup$ ""Then once the reaction starts, the neutron poison burns away quickly, "" this is the wishful thinking part of your idea. $\endgroup$ – Georg Feb 14 '17 at 20:07
  • 1
    $\begingroup$ I don't think there are any stable nucleii with that high an absorption cross section, although there are trans-uranic isotopeswith very high neutron absorption cross sections that would then rapidly undergo fission (but that would spoil you intent). At any rate, once the reaction starts, you want as high a rate of netrons available for fission as possible, since the energy released is dictated most strongly by the exponential growth of the reaction; capturing neutrons reduces the factor in the exponent. $\endgroup$ – Mark Fischler Feb 14 '17 at 21:24
  • 1
    $\begingroup$ What you can do, instead, is super-compress the fissile material by the shock was from a fusion device. That is how multi-megaton Fission-Fusion-Fission bombs work $\endgroup$ – Mark Fischler Feb 14 '17 at 21:24
  • $\begingroup$ The question is inspired by an analogy to thermal reactors in which the burnup of traces of xenon 135 can cause sudden increases in reactivity, because xenon 135 has a thermal neutron absorption cross section of 300,000 barns, so it burns up 500x faster than the fuel. But there doesn't seem to be anything with a fast neutron cross section orders of magnitude higher than U-235 or Pu-239. $\endgroup$ – Jonathan Ray Feb 23 '17 at 19:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.