You can breed weaponizable fissile material in fast reactors. But by far the easiest way to make bomb material in a reactor is what they did in the Manhattan Pr

You can breed weaponizable fissile material in fast reactors. But by far the easiest way to make bomb material in a reactor is what they did in the Manhattan Project, using graphite moderated thermal neutron production reactors (not fast reactors). And it's even easier still to just build gas centrifuges and enrich uranium without needing a reactor at all. Becoming a nuclear superpower weeks after operation of a fast-neutron reactor is a bit of an overstatement.