The world needs a new source of energy, an unspillable source.

Are nuclear-powered ion drive engines worth using in outer space?

A few years back,Nasa considered sending such a probe to Jupiter.It would have been called JIMO(Jupiter Icy Moons Orbiter).It would have been powered an ion drive.With a Fission Reactor as well, It would have done a lot more good science due to the extra fuel onboard. How about a fusion-ion drive even. What would be the benefits of such an engine design?What things could U do with such a spacecraft then?How fast could it go?