system
Given the following post, respond as if you're commenting on Reddit:
human
How is nuclear fission controlled in atomic weapons? What’s stopping the fission process from ongoing and continuously creating the reaction indefinitely?
gpt
>How is nuclear fission controlled in atomic weapons? 

It's not really controlled, other than the fact that you want the system to be subcritical until you're ready to produce yield, and then you want the system to become as deeply supercritical as possible when you set it off.

>What’s stopping the fission process from ongoing and continuously creating the reaction indefinitely?

The weapon physically blows itself apart, spreading the fissile material out, and making it subcritical again. (And furthermore, the fuel isn't infinite; even if you could cause every single fissile nucleus to undergo fission, it still wouldn't go forever.)