[BAReF00t]

Who said anything about uranium?

There are elements with far more favorable decay paths. Short decay + using that decay too = pretty much a clean nuclear reactor.

[Symmetry]

Well, I'm no nuclear engineer but I'd hope that if you could make one of those that was light enough NASA would use that instead.

[greglindahl]

It's called an RTG, and they've been using them for decades.