3 ms·

> But had one of them exploded, the other one would have as well Really? Because everything we have been told about nukes so far is that this does not happen.

pja 4y ago
The mechanism - neutron flux from the first bomb pushing the second into criticality - seems plausible. Where does it say that this is impossible?
- mnw21cam 4y ago
  Criticality doesn't work like that. A lump of fissile material either is or isn't supercritical, regardless of the neutron flux. There are things that can change a lump from subcritical to supercritical, like lowering its temperature so it contracts, moving two lumps together, or using conventional explosives to compress the lump, but throwing a load of neutrons at it is not one of them.
  - pja 4y ago
    No, you’re right & I wasn’t thinking it through. At worst it will cause the core to fizzle due to radiation induced decays, but it shouldn’t detonate. Still not great of course! Now you’ve spread the second bomb’s worth of Pu or U over a wide area with the energy of the first one. Toxic /and/ radioactive. What’s not to like! (I may have been digging up a partial memory of muclear missile fratricide by Pu predetonation during compression due to the extra neutron flux, which IIRC is a real thing, but would require the second warhead to go through a full conventional explosion.)
dontwearitout 4y ago
I'm not an expert, but my understanding is nukes are designed to not detonate if they are damaged or destroyed by a conventional explosion. But if they're sitting next to another nuke which detonates, the core doesn't even have to go critical for it to release a bunch / most of its energy due to its neighbor's neutrons flux. Similar to how in thermonuclear (fusion) weapons a simple blanket of unenriched uranium can provide a huge boost to the weapon output.
- mnw21cam 4y ago
  I think for two nukes sitting any further apart than a few metres, if you set one off, the fissile material in the second nuke may have some small number of fissions due to incident neutrons, but it would be a very long way away from "most" of its energy. Just from a physics point of view, let's be generous and say that a single fission releases four neutrons, and I'm pretty sure that's an overestimate. If the fissile material in the second nuke has a radius of 0.1m (r), and sitting 5m (R) away, then it would be exposed to at a maximum πr^2 / (4πR^2) = 0.0001 of those neutrons. If we imagine that every single one of those neutrons causes a fission in the second nuke (and that won't be true either) then the yield from the second nuke will be less than a thousandth of the yield from the first nuke. Secondary fissions from the neutrons released from those fissions may increase this a little, but the second nuke is subcritical, so it won't be by much. I'd love to hear from someone who actually knows the answer for sure, though.

pja 4y ago

The mechanism - neutron flux from the first bomb pushing the second into criticality - seems plausible. Where does it say that this is impossible?

mnw21cam 4y ago
Criticality doesn't work like that. A lump of fissile material either is or isn't supercritical, regardless of the neutron flux. There are things that can change a lump from subcritical to supercritical, like lowering its temperature so it contracts, moving two lumps together, or using conventional explosives to compress the lump, but throwing a load of neutrons at it is not one of them.
- pja 4y ago
  No, you’re right & I wasn’t thinking it through. At worst it will cause the core to fizzle due to radiation induced decays, but it shouldn’t detonate. Still not great of course! Now you’ve spread the second bomb’s worth of Pu or U over a wide area with the energy of the first one. Toxic /and/ radioactive. What’s not to like! (I may have been digging up a partial memory of muclear missile fratricide by Pu predetonation during compression due to the extra neutron flux, which IIRC is a real thing, but would require the second warhead to go through a full conventional explosion.)

dontwearitout 4y ago

I'm not an expert, but my understanding is nukes are designed to not detonate if they are damaged or destroyed by a conventional explosion. But if they're sitting next to another nuke which detonates, the core doesn't even have to go critical for it to release a bunch / most of its energy due to its neighbor's neutrons flux. Similar to how in thermonuclear (fusion) weapons a simple blanket of unenriched uranium can provide a huge boost to the weapon output.

mnw21cam 4y ago
I think for two nukes sitting any further apart than a few metres, if you set one off, the fissile material in the second nuke may have some small number of fissions due to incident neutrons, but it would be a very long way away from "most" of its energy. Just from a physics point of view, let's be generous and say that a single fission releases four neutrons, and I'm pretty sure that's an overestimate. If the fissile material in the second nuke has a radius of 0.1m (r), and sitting 5m (R) away, then it would be exposed to at a maximum πr^2 / (4πR^2) = 0.0001 of those neutrons. If we imagine that every single one of those neutrons causes a fission in the second nuke (and that won't be true either) then the yield from the second nuke will be less than a thousandth of the yield from the first nuke. Secondary fissions from the neutrons released from those fissions may increase this a little, but the second nuke is subcritical, so it won't be by much. I'd love to hear from someone who actually knows the answer for sure, though.