You'd have to go far more advance for an efficient engine thn a nuclear reactor. You'd need something in the line of an Matter/Anti-matter reactor. They would be relatively light and give massive energy. 1kg matter and 1kg anti-matter would give a reaction equal to about 47 megatons of TNT. And yes, that is quite a lot. The problem at the moment is though that it is incredibly hard to create anti-matter. It probably takes a hundred more times more energy to create one anti-proton than you'd get by joining a proton and anti-proton. Then there is to problem of the storage of the anti-matter, it's hard to store unless it's inside a special magnetic field. Then again, anti-protons tend to go haywire, due to their negative nature, when inside the field. So you'd need Anti-Hydrogen, a neutral anti matter, for that...which is even harder to create.
Well you get the point. We are not nearly close enough to put stuff like this to good use in space.