I agree that nuclear batteries would be a great idea in theory. I'm not a NASA employee, or scientist, but I would venture a guess that the risk is not worth the reward just yet. Imagine the first time one of those suckers breaks apart on liftoff.... "We're sorry ladies and gentlemen, Florida is now closed. We hope you enjoyed your stay and we look forward to seeing you when we reopen in 30 years. Thank you and goodbye."