True, if intelligent beings needed the fissionable material to survive, the material being 4.6 billion years old is going to mean most of the radioactive isotopes have nearly all converted to lead or to a more stable isotope by now. It would have been good pickings 4.6 billion years ago, but now things are quite run down. So if you reckon you need fissionable material. Where are you going to get that? A comet, might be a good source of heavy water but you need equipment to process that. So are you driven back into a gravitation well to get the metal for the equipment in the first place?