system
Given the following post, respond as if you're commenting on Reddit:
human
How do nuclear reactors/atomic bombs prevent a fission reaction before it is wanted?

I know how fission reactions work (to a certain extent atleast), but how are the neutrons contained and prevented from starting a reaction at any random point in time? Shouldn’t a reaction start the second any neutron at high enough energy hits the nucleus of another atom, and therefore be completely unpredictable?
gpt
A few nuclear reactions does not a bomb or reactor make. You need a _lot_ of them arranged in a chain reaction (critical means that each reaction creates another on the average; supercritical means that each reaction creates more than one other reactions on the average) for it to be interesting on a human scale — billions of them. If only tens of thousands of reactions happen, nothing will even get warm, much less explode. (To put it into perspective, you would need about 150 _million_ uranium-235 atoms to fission to release the equivalent energy of a microgram of TNT.)

You can stop the chain pretty easily in a lot of ways. In a reactor, for example, you can put control rods (neutron absorbers) inside of the assembly, which keep it subcritical (you'll never get a long chain of reactions if a substantial portion of those neutrons are getting eaten by the rods, in essence). You can then raise the rods, start the reaction (making it briefly supercritical) and then level it off at the power level you want (critical). There are other physics and engineering tricks that can modify the critical conditions of a reactor as well (e.g., some reactors, like the TRIGA reactors used for research, are designed with materials that absorb neutrons as they get hotter — so in such a reactor, the reaction will automatically level off or decrease without any additional interference from an operator, once you start it up).

For a bomb, you start with a system that is subcritical (not configured to sustain a chain reaction), and you make it supercritical in an instant. The "instant" aspect is important: the bomb is going to destroy itself, so you want it to get as many reactions as possible before it does so. The two main ways to do this with fission weapons are the gun-type approach (take two subcritical masses and shoot them together, making a briefly supercritical mass), and the implosion approach (take a subcritical mass and use carefully-engineering high-explosives to symmetrically decrease its overall volume by a factor of 2 or so, making it twice as dense and supercritical). As with the reactor example, there are many physical and engineering tricks to both keep it from going off when not desired (for example, some designs from the 1950s and 1960s had neutron-absorbing strips in the cores that would be removed shortly before use; they were there to make sure there was no possibility of an accidental detonation), and to make sure that when it is desired to explode, it does so very well (such as having a bunch of neutrons injected into the core at just the right moment, kick-starting the chain reaction off).