[Flyndaran]

If you're going to go with that much more uranium, then there could be natural reactors in the present like the one earth had over a billion years ago. None now due to that pesky half life disallowing sufficient densities without enrichment.
Gurps Disasters: Meltdown And Fallout mentions how it's just possible for low tech peoples to create simple reactors this way.
Slaves work the uranium mines to keep the Roman Empire alive... until the evil X invent the slightly superscience version of nukes.
A real honest to goodness dark age without the need to handwave "lost" technology and knowledge. They just never had it.