In space, the fission doesn't need containment, or even much shielding if it is far enough away from the rest of the ship.

In the cloud tops of Titan, Venus, Saturn, Uranus, or Neptune, a nuclear power plant may be as simple as a naked pile suspended near the bottom of a big fabric tube with wind turbines at the top. The radioactively-heated, buoyant air inside holds it up, and, escaping through the turbine, generates power. (Enough is held back so it stays up.)

Perhaps surprisingly, gravity is almost Earth-normal on all four planets. Perhaps more surprisingly, the temperature at a not unreasonably-high pressure is also close to Earth-normal (except on Titan). But getting back to orbit from any of the last three would be very, very difficult, so such a reactor would only ever power robots.

Getting back to orbit from Titan is easy. From Venus, it is pretty hard; and once in orbit you would need to refuel to get anywhere else.