Nuclear fuel is costly, rare and creates the potential for disasters. There are more efficient theoretical methods for conventional cosmic propulsion such as ion engines. The point is really moot since once you're in outer space, you only really need thrust to break away form gravity, speed up, slow down, change direction, or stop. Most of those things could be done in small bursts unless you actually want to be constantly accelerating. That's risky since you may not have enough power to stop, your mass and density will be constantly increasing while relative time slows down.
This is why I think we need to get away from conventional propulsion for manned and unmanned space flights and start to more seriously research warping space, stable wormholes, and what causes mass and whether that particle can be manipulated.