[Vecr]

Voyager uses a RTG. The USSR used some full-on nuclear reactors in space, but as far as I know no one else publicly did.

Edit: Yep, not sure how I forgot about that.

[joeyo]

The US put a fission reactor in space too. They did it first, in fact: https://en.wikipedia.org/wiki/SNAP-10A

Edit: it’s apparently still there and will be for a long time (!) albeit non-functional:

  > Decay date: April 3, 5966 (planned)