If this civilization collapses, how are we going to get to the point where we can build nuclear fission reactors?

Is it that hard to enrich isotopes enough to make a steam engine that runs on radium, thorium or uranium instead of coal?

If you know that radioactive isotopes exist, it sounds like steampunk, 1850s, level tech could build a power plant, but perhaps not a bomb.

I’m not qualified to answer, but there was a bit of discussion about the idea on Reddit:

https://www.reddit.com/r/worldbuilding/comments/57peek/can_y...

Interesting.

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.

Controlled cold fusion would be even more neater as it enables you to synthesise arbitrary molecules out of just hydrogens.