I wonder if it would be possible to take a radioactive material, one that is really hot, like Plutonium 238. You put that inside a sealed container which has a synthetic diamond rod pushed up against it as the heat transfer medium, this can then be used to power a stirling engine.
I guess it isn't that much different from a regular nuclear power plant, just in this case the heat is used to power a stirling engine instead of make steam for a turbine.
Whatcha think?