If this civilization collapses, how are we going to get to the point where we can build nuclear fission reactors?
If you know that radioactive isotopes exist, it sounds like steampunk, 1850s, level tech could build a power plant, but perhaps not a bomb.
https://www.reddit.com/r/worldbuilding/comments/57peek/can_y...
I don’t think it’s that hard to build a vessel full of warm radioactive pebbles to boil water.
Sorting ore by chemical element, and then making a fluid / gas / solution of the fuel element to sort by isotope mass in a centrifuge may or may not be doable for “blacksmith” level technology, though. Hard to say.