Is a Moderator Used in Nuclear Bombs?

[paweld]

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.

 

[mgb_phys]

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.

 

[Astronuc]

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.

 

[mathman]

Implosion is necessary for Pu239 bomb, but not for U235 bomb.

 

[Sakha]

Some bombs use beryllium or tungsten carbide neutron reflectors.