Just to be An Explainer Guy, the mechanism for making a nuclear bomb explode is genuinely just putting the two halves together. The reaction through the material goes up by a factor of about 2.5 every "shake", which is about 10 nanoseconds. Depends on how wide the core is.
So the initial reaction is 200 MeV, or about 1/30th of a nJ, but then 10 nanoseconds later, it's 1/10, then 1/3, then 1, 3, 9, 27...
If the core could just stay together for a full fucking second, the last reaction would give off 3^100,000,000 power more energy than the initial. That's like, (I think?) a 10^47,712,125 multiplier. I cannot emphasize how stupidly large that number is. That is a googolplex, times a googolplex, times a googolplex, times 10^50. I am too lazy to put that kind of number into physical terms, but it would not surprise me if it was trillions of times bigger than the big bang.
But real life has limitations. First, you know, to have a supercritical reaction go on for 1 second would take an absolutely ludicrous amount of uranium, but outside of that kind of bullshit, there's just. Like. It's trying to explode apart very quickly. And we can hold it together shockingly well for a bit, but it really wants to explode apart, and once it explodes apart the nuclear reaction stops. Those two halves must be touching for the magic to keep happening.
There are huge rewards for keeping the core together just a smidge longer. Every ten nanoseconds gives off triple the power of the previous ten, so they're quite motivated. But! Fuck! Is it hard to keep a nuclear explosion small! The "easy" method bombs used to use was to like, fire one half of the core at the other half. Like with a cannon. A cannon inside the bomb. But that just couldn't keep the reaction going long enough, so the better method was detonating a bunch of very precisely timed plastic explosives all around the core, so that the blast waves would squinch it together just a bit longer and that worked out pretty good. Multistage bombs use alternating nuclear blasts to squinch the next blast together, and that works out pretty good because damn near the only thing that can cancel out a nuclear blast for any amount of time is another nuclear blast, to say nothing of how fusion works with all of this.
It's nifty is what I'm saying. I think it's neat.