[evo]

I disagree; under normal circumstances regarding the HE explosives you are 100% correct but I suspect a nuclear shockwave at near point blank range might be fast enough to push the near side of the primary pit to the far side in a time interval short enough to cause an additional criticality. Probably not to design yields, but plausible.

[j9461701]

A nuclear chain reaction takes place roughly over a single microsecond. To travel the ~11 inches of a bomb, in ideal circumstances, a high explosive needs roughly 200 microseconds. It is not impossible that the exact right stuff could happen to trigger the bomb, but it is extremely unlikely - it took the smartest minds in america working together for 3 years to figure out the exact precise timing to prevent a nuclear fizzle and achieve a true atomic explosion. It is, contrary to pop culture, near-impossible to pull off by dumb luck. Mostly you'll just make a radioactive mess.

[evo]

Right! But that's talking about compressing a plutonium pit in atmospheric pressure--being an extremely dense metal, plutonium will preferentially "squirt out" in any direction rather than compress, if there's a direction it can go in. Therefore, you need to make a spherical shockwave via explosives.

But that's in 1 atm! The initial wavefront of a fission device is going to be conservatively about 4 inches a microsecond (based on early above-ground test photos, modern high-yield devices would probably be faster still). This could very well turn the entire physics package of the secondary weapon into a thin pancake on the blast front, the plutonium can't get out of the way fast enough to avoid compression. This is a totally different scenario than a one-point-safety fizzle. The HE explosives in the second bomb might as well not exist relative to the overpressures faced from the first bomb's detonation.

[pletnes]

Another point is the massive neutron flux, which will get some nuclear reaction out of the bomb material in and of itself. Neutrons will set off both fission and fusion reactions, simultaneously.

[arethuza]

I wonder if you had two bombs in a confined space whether the X-ray flux from one would cause a radiation flux driven implosion in the other - similar to how the primary causes the secondary to implode in an H-bomb.

Almost certainly not!