Obtainment/procreation of weapons grade uranium. Conceptually speaking, nuclear bombs are simple. You’re creating a chain reaction with the enriched uranium by effectively smashing it together at a high speed. Fat man/little boy both had smaller explosives inside of them that were designed to set off the chain reaction required for the nuclear explosion.
Basically, you have to shave 3 neutrons off of naturally occurring uranium. This changes it from u238 to u235. This is a less stable version.
This is a very lengthy process that involves very specific and expensive and large equipment
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