Hmm, I see... well, just consider it if you want to be more real-robot-ish.
Basically like this, but with smaller reactor (yes it's real)
1. Nuclear reactor (in this picture, fission)
2. heat exchanger pipes
3. whatever engine you wanted to use (in this picture, turbojets)
3a. Reaction mass injector if necessary.
4. profit
Project documentation for great justice
Running out of juice, call it a nite guys.