system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are nuclear weapons so hard to develop if the technology for them has existed for 70 years?
gpt
The hard part is getting enough weapons-grade uranium or plutonium, which is actually quite labor intensive and expensive to do, unless you have nuclear reactors - which many countries don't.

Japan could build a nuclear bomb within a month if they wanted, and possibly sooner. So could any other country with nuclear power.

This is why people are iffy about letting developing countries build nuclear reactors - they can be dual use into making materials for weapons.

The other issue is delivery systems, like rockets/missiles, technologies most countries lack, and which are, again, fairly difficult to design/build, and have to be produced to high precision.

There are instructions for how to build a nuclear bomb online, but good luck getting 10 kg of weapons material to build one with.