3 ms·

i wonder what engineering breakthroughs would be required to build a typical single staged implosion style nuclear weapon with the explosive yield of your typic

i wonder what engineering breakthroughs would be required to build a typical single staged implosion style nuclear weapon with the explosive yield of your typical fire cracker. The pit would be very very tiny and then the HE lensing and detonation circuitry would also have to be very tiny. Assembling it would be harder than assembling the movements in a watch I bet.

AnimalMuppet 2y ago
There's this thing called "critical mass". Below that, you do not get an explosion, and no engineering breakthrough is going to change that. Per Wikipedia, the smallest critical mass is 2.73 kilograms, for Californium 252. For that small a yield, you'd want just barely more than the critical mass. But: 1. The yield is probably a very sensitive function of the mass. It's going to be hard to get the yield to be both greater than zero and less than, say, a hand grenade. 2. You need explosives to make the implosion happen. Those explosives will almost certainly themselves have yield higher than a firecracker.
- lazide 2y ago
  Critical mass is a function of neutron absorption cross section, neutron emission (during fission) properties, neutron reflection rates, and density. Those critical mass numbers are assuming either STP or ‘not abnormally insane’ density levels and little to no neutron reflectivity or moderation. But production nuclear weapons already get below what we’d consider critical mass by using density improving tricks (with the explosive lenses), neutron reflectors, initiators, etc. Still, anything less than a kg would require some truly amazing tricks, and ‘firecracker’ sized would be likely microgram sized fissile material samples. Which yeah, good luck.
  - klyrs 2y ago
    Rephrasing the question for amusement: how many kilotons of TNT would it take to compress a gram of plutonium to criticality?
    - lazide 2y ago
      A good xkcd question! My guess would be so much that the actual criticality event would be caused by the gram of plutonium smashing into the surface of the resulting TNT neutron star at some high fraction of c. Edit: found this nice write up on Wikipedia [https://en.m.wikipedia.org/wiki/Tamper_(nuclear_weapon)#CITEREFReed2011 https://en.m.wikipedia.org/wiki/Tamper_(nuclear_weapon)#CITE...] which refers to some papers/books which appear to show that theoretically, with the right reflector/tamper, up to an 8 fold reduction in critical mass is possible. Which would, I guess, make Plutonium’s critical mass (uncompressed) go from 11kg to 1.375 kg. Throw in the neutron amplification effects of beryllium with high radiation exposure, hyper compression using some of the new super-high-detonation velocity chemical explosives, and I’d bet there are production nukes out there with sub-kg plutonium pits. Cool beans. Probably still in the 900g range though.
      - euroderf 2y ago
        Yield in kT ?
lazide 2y ago
It wouldn’t be a nuclear explosion, but a fizzle. too far below critical mass, unless we’re talking ‘hyper compressed collection of 5-6 digit number of uranium/plutonium atoms’ Which at the point you’re using hohlraums and igniting the collection with petawatt class lasers, I’m not sure ‘firecracker’ levels of measurements have any relevance anymore.
saulpw 2y ago
It's not possible. There is a minimum critical mass, which can be reduced with reflectors etc, but not down to even a 1t TNT yield.

AnimalMuppet 2y ago

There's this thing called "critical mass". Below that, you do not get an explosion, and no engineering breakthrough is going to change that. Per Wikipedia, the smallest critical mass is 2.73 kilograms, for Californium 252. For that small a yield, you'd want just barely more than the critical mass. But: 1. The yield is probably a very sensitive function of the mass. It's going to be hard to get the yield to be both greater than zero and less than, say, a hand grenade. 2. You need explosives to make the implosion happen. Those explosives will almost certainly themselves have yield higher than a firecracker.

lazide 2y ago
Critical mass is a function of neutron absorption cross section, neutron emission (during fission) properties, neutron reflection rates, and density. Those critical mass numbers are assuming either STP or ‘not abnormally insane’ density levels and little to no neutron reflectivity or moderation. But production nuclear weapons already get below what we’d consider critical mass by using density improving tricks (with the explosive lenses), neutron reflectors, initiators, etc. Still, anything less than a kg would require some truly amazing tricks, and ‘firecracker’ sized would be likely microgram sized fissile material samples. Which yeah, good luck.
- klyrs 2y ago
  Rephrasing the question for amusement: how many kilotons of TNT would it take to compress a gram of plutonium to criticality?
  - lazide 2y ago
    A good xkcd question! My guess would be so much that the actual criticality event would be caused by the gram of plutonium smashing into the surface of the resulting TNT neutron star at some high fraction of c. Edit: found this nice write up on Wikipedia [https://en.m.wikipedia.org/wiki/Tamper_(nuclear_weapon)#CITEREFReed2011 https://en.m.wikipedia.org/wiki/Tamper_(nuclear_weapon)#CITE...] which refers to some papers/books which appear to show that theoretically, with the right reflector/tamper, up to an 8 fold reduction in critical mass is possible. Which would, I guess, make Plutonium’s critical mass (uncompressed) go from 11kg to 1.375 kg. Throw in the neutron amplification effects of beryllium with high radiation exposure, hyper compression using some of the new super-high-detonation velocity chemical explosives, and I’d bet there are production nukes out there with sub-kg plutonium pits. Cool beans. Probably still in the 900g range though.
    - euroderf 2y ago
      Yield in kT ?

lazide 2y ago

It wouldn’t be a nuclear explosion, but a fizzle. too far below critical mass, unless we’re talking ‘hyper compressed collection of 5-6 digit number of uranium/plutonium atoms’ Which at the point you’re using hohlraums and igniting the collection with petawatt class lasers, I’m not sure ‘firecracker’ levels of measurements have any relevance anymore.

saulpw 2y ago

It's not possible. There is a minimum critical mass, which can be reduced with reflectors etc, but not down to even a 1t TNT yield.