It's been researched in the 80s/90s. The best solution was a gas-dynamic laser with a Po-220 (the activity of U238 within its entire 10k years halflife packed into a time of two weeks) reservoir as a heat source.
Oh thanks, I've looked into it again and it seems it's highly controversial. I mean, even if it wasn't a hoax, the design is pretty much infeasible if you need a heat source that you can only store for less than a week and that'll kill everyone within line of sight if its container is breached.
