Breeder reactors, as others have pointed out. Oklo is a modern design based on the concept: https://www.nrc.gov/reactors/new-reactors/col/aurora-oklo.ht...

Also see “Problem free nuclear power and global change”: https://www.osti.gov/biblio/614877

And “Nuclear fission power for 21st century needs”: https://www.sciencedirect.com/science/article/abs/pii/S01491...