It's about as realistic as propelling the vehicle with unicorn farts. In particular, the kinds of nuclear propulsion being discussed in this thread could not do it. Solid core nuclear thermal rockets using hydrogen have an Isp of about 1000, so they could accelerate a vehicle at 1 gee for less than an hour.

The power/weight ratio of nuclear rockets actually sucks, compared to chemical rockets. Conveying heat through a solid/fluid interface is awkward and slow compared to just making it in situ by combustion.