What stops a nuclear bomb’s chain reaction?

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What stops a nuclear bomb’s chain reaction?

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Can’t really stop a nuclear bomb, it stops when it runs out of useable fuel. In a nuclear reactor the emissions are controlled with graphite rods that absorb the neutrons

You simply keep the fissile material below critical mass, usually by keeping it in parts that are sufficiently separated or by having it be below critical density to begin with.

To start the chain reaction you either shoot one part into the other, bringing it to critical mass, or put the sub-critical material under massive pressure to condense enough that it becomes critical.

Nuclear bombs are bombs because the reaction is uncontrolled – a lot of fission happening exponentially. Once it runs out of fuel (because most has been fissioned already, or because it’s too far for neutrons to reliably hit), well, there’s fewer and fewer atoms neutrons can hit, so nothing to create a chain. If your question is what stops the fission of products of reaction, it’s mostly the latter (and the fact that some of the products will eventually turn stable).

Either it runs out of fissionable material or that material is pushed away fast enough to avoid being set into a chain reaction.

The temperature in and around might still be hot enough to cause fission, however it is harder and doesn’t release enough energy to sustain itself.

After it has begun? The core undergoes a prompt-critical chain reaction which either fissions all of the available fuel, or which stops when the resultant explosion blows apart the core, preventing fission neutrons from colliding with fissionable nuclei and halting the chain reaction. This is why some weapons have lead or depleted uranium tampers surrounding the core, in an effort to increase fission yield (% of core mass consumed by the chain reaction) by keeping it contained even a few microseconds longer.

We call that “chain reaction” fission. It is actually much like any other bomb. It either burns through every radioactive atom it can or the force and heat generated by the explosion push the atoms too far away to sustain the runaway fission. In fact, it is estimated that only 1kg of the 64kg of radioactive material in the Little Boy bomb dropped on Hiroshima actually underwent fission.

That’s right. Only about 1.5% of the enriched uranium was actually consumed by the reaction that made a 15 kiloton explosion that immediately killed approximately 66,000 individuals. Newer hydrogen fusion bombs are even more efficient.

There are 2 (primary) types of Nuclear bombs: fusion and fission (I’m ignoring neutron bombs or dirty bombs).

Fission bombs are propagated until the fissionable material is too diluted by the distance between the nuclei of the atoms for the chain reaction to continue. In essence the bomb runs out of fuel because it has blown the material too far apart for the neutrons to hit the other fissionable material.

Fusion bombs can be much more complicated than I’m laying out here and can actually be multiple layers, but leaving aside engineering complications, the bomb continues to explode until the small atoms no longer have the energy to fuse together. Unlike in fission there is a huge energy barrier to fusion, you can keep fusible material in very close contact without any real chance of producing fusion. The energy barrier to induce fusion for less ideal fusion material (like the air, thank your preferred deity of choice – I suggest Tod) is great enough that fusion quickly hits a net loss of energy point soon after the initial blast.

To put it politely, it disassembles itself. Nuclear chain reactions require what is known as a critical mass. Basically, there has to be a specific amount of fissile fuel in a specific volume for the chain to occur. Each reaction in the chain releases at least two particles as it’s final step, as well as energy. These released particles run into other particles, moving them slightly. Since this happens all throughout the critical mass, it expands as the chain progresses. Eventually, enough reactions and collisions have occured so there is no longer a critical mass of fuel.

1. It runs out of fuel, i.e. there aren’t anymore atoms to split.

2. The (unsplit) atoms become so spread out after the blast they aren’t close enough to hit each other.

People on here are right that the bomb “blows itself apart,” but there is a detail that helps understand it a bit closer. If you could see the reaction in the fuel core in super slo-mo (like, on the scale of nanoseconds), you’d see that as the reaction spread through the core, the core itself heats up. As it heats, it expands. At some point, the expansion is large enough that the neutrons that spread from reaction to reaction won’t be able to find another atom to react with, on average, and the overall criticality of the system — the ability for one splitting atom to create splits in more than one other atoms — will drop. Once it dips too much, the reaction is over, and whatever energy you’ve gotten out is what you’re going to get.

All of this happens on the scale of tens of nanoseconds. A lot of bomb design is about trying to keep that core from expanding too much, because the longer you keep it together — and again, “longer” here means tens of nanoseconds — the more explosive energy you get. [Here is a diagram](http://blog.nuclearsecrecy.com/wp-content/uploads/2014/11/Glasstone-Snowplow-region.jpg) from the 1950s that illustrates the core expansion issue, from a once-classified manual about nuclear weapons, which illustrates the use of a heavy “tamper” (just a heavy metal around the core) that tries to slow down the expansion for a tiny bit of time (creating a “snowplow” region where the core expands into the tamper and squishes it).