A fission plant can be extremely cheap in certain environments. For example, in the cloud tops of Venus, Saturn, Uranus, Neptune, Ganymede, Titan, or Triton, a fission reactor is as simple as a big fabric tube suspended from a balloon, with a naked atomic pile hanging near the bottom, and a wind turbine at the top. Radiation is absorbed by the air inside the (sufficiently broad) tube, which rises through the turbine at the top. You could dispense with the balloon if you constrict the exit aperture just right.