Project Pluto is a ramjet, needs air to function. Only benefit over conventional hydrocarbon fueled engine/ramjet is greatly increased loiter time.

To my knowledge the only proposed "nuclear rocket" idea capable of achieving orbit or beyond would be project Orion:

http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29

It throws out coke can sized nuclear bombs underneath itself in quick succession, that thing would win any dick waving competition hands down!