Building a nuclear warhead requires assembling a large sphere (ish) of material well above critical mass faster than it can explode. Only then does the chain reaction efficiently consume a lot of fuel before it breaks up (inertial confinement). Usually, you need to use explosives to blast the different pieces of fuel into each other. This requires very accurate control of the explosions that ignite the warhead, and is where the secrets are.

A nuclear reactor uses fuel juuuust barely above critical mass so the reaction is slow. It's a very different thing. Producing the fuel is just basic centrifuge isotope separation, which has many non-fission applications.