Demon core from Fat Man/Little Boy

[Your_Great_Darsh_Face]

@Raguleader

IANANuclear physicist nor a weapons designer, so I might be entirely wrong here, but my understanding is that there is not enough plutonium in the weapon core for it to go critical, even with the neutron reflecting material that may be around it. When the conventional explosives in the warhead go off all around the core, they compress the whole thing momentarily, putting it under considerably higher pressure, which makes the lower mass of plutonium dense enough to react energetically.

Basically, a wee bit of plutonium isn’t putting out enough neutrons (protons? It’s been years since I took a high school science class) to create a chain reaction. Like a small pile of nitrate fertilizer, it’ll just sit there. Get enough of it together and it’ll fizzle. Get way too much of it in one pile and it’ll go bang. Set a smaller amount of it up in a warhead with a detonator designed to go off more easily in a specific controlled fashion, and it’ll go bang much more easily at the time and place of your choosing.

Mind you, the whole thing is designed to only work if it goes off just right. If it malfunctions, or if the vehicle transporting it is in a crash, the bomb won’t go off in a huge blast, it’ll just make a nasty hazmat mess in the vicinity of the crash but otherwise leave the area intact. The fact that the warheads are designed to fail-safe so easily is also why it used to be (and may still be) the practice to have several warheads earmarked for each target. If your only nuke that hits the Kremlin fails to go off, that’s embarrassing. If one nuke out of fifteen that you drop on the Kremlin fails to go off, it’s not like you were worried about overkill to begin with.

NB - “Neutrons” was right - plutonium, like U235, will fission when hit by a neutron and in the process produce more neutrons to do the same to other nuclei, hence the chain reaction. The full answer would take longer.

[engineer_comp_geek]

Moderator Note

@FXMastermind

I was trying to mislead about the design of an H bomb.

Let’s not have any intentional misleading in GQ, please. That comes off as trolling or intentionally being a jerk.

No warning issued.

[Machine_Elf]

@Velocity

Pardon my ignorance, but why was the plutonium ball in front of Slotin able to go critical simply by putting a beryllium shell (a neutron-reflector) over it, thus making it go from subcritical to critical, but the plutonium ball in a nuclear weapon is still subcritical even though it’s surrounded by neutron-reflecting material?
Isn’t the plutonium core in the same size and shape in both cases?

The short answer is no. Raguleader pretty much covered it. More detail here:

@Wikipedia

The Fat Man device specifically used 13.6 lb (6.2 kg, about 12 fluid ounces or 350 ml in volume) of Pu-239, which is only 41% of bare-sphere critical mass.…Surrounded by a U-238 reflector/tamper, the Fat Man’s pit was brought close to critical mass by the neutron-reflecting properties of the U-238. During detonation, criticality was achieved by implosion.

That implosion is the really difficult part. The plutonium pit is surrounded by high explosive arranged in a very specific configuration, with detonators placed at very specific locations on the outer surface. The electronics are designed to trigger all of the detonators with exceptionally precise timing. If the timing is off, then the pit is not squeezed on all sides simultaneously; it just gets displaced toward whichever area detonated late. So if a crash happens to somehow trigger some of the high explosive on one side of the pit, you won’t get a nuclear detonation; you’ll just shoot the pit out to the other side.

In addition to fissile material being hard to come by for the average terrorist, the technology required to turn that material into a nuclear weapon is also deliberately hard to come by.

[bonzer]

I’ll incidentally note that the origin of John Cusack’s bomb design diagram in Fat Man and Little Boy has just been fingered.

[Precambrianmollusc]

There is an excellent book “Oppenheimer shatterer of worlds” by Peter goodchild, it was released in conjunction with a BBC series about Oppenheimer. It has some excellent discussions of the experiments and SNR of the physics of a nuclear bomb

[Velocity]

Pardon my layman ignorance, but if the core was emitting neutrons, and every atom nucleus has a finite number of neutrons, then would a core that has gone critical eventually deplete itself of neutrons, leaving only protons and electrons behind? And with “neutron reflectors” - are the neutrons really being spat out by the core and flying out and striking the reflector?

[beowulff]

@Velocity

Pardon my layman ignorance, but if the core was emitting neutrons, and every atom nucleus has a finite number of neutrons, then would a core that has gone critical eventually deplete itself of neutrons, leaving only protons and electrons behind? And with “neutron reflectors” - are the neutrons really being spat out by the core and flying out and striking the reflector?

No.

PU-239 emits neutrons because it undergoes spontaneous fission - an occasional atom decides to split into smaller pieces, and the neutrons are left over. If one follows this chain of decay, it eventually leads to Lead. And Lead has lots of neutrons in it.

ETA: Yes, the reflectors are designed to re-direct (bounce) neutrons back to the core, reducing critical mass.

[buddy431]

@Velocity

Pardon my layman ignorance, but if the core was emitting neutrons, and every atom nucleus has a finite number of neutrons, then would a core that has gone critical eventually deplete itself of neutrons, leaving only protons and electrons behind? And with “neutron reflectors” - are the neutrons really being spat out by the core and flying out and striking the reflector?

Yes, radioactive materials decay over time (it will never “run out of neutrons”, but it will run out of Plutonium).

The half life is about 24,000 years, so it’s not going to "run out’ on human time scales.

However, the radioactive decay is a pretty big component of the aging process of the core. It releases gas which can cause bubbles to form. The buildup of fission products can change the material properties of the core.

[Your_Great_Darsh_Face]

@Velocity

Pardon my layman ignorance, but if the core was emitting neutrons, and every atom nucleus has a finite number of neutrons, then would a core that has gone critical eventually deplete itself of neutrons, leaving only protons and electrons behind? And with “neutron reflectors” - are the neutrons really being spat out by the core and flying out and striking the reflector?

Not yet mentioned, but no decaying nucleus will ever spit out all its neutrons, leaving only protons and electrons behind. To do that it would have to fission down all the way to hydrogen (the only no-neutrons element), and that doesn’t happen.