Space

It is theoretically possible to use the nuclear fuel itself as the propellant too in various ways, and they can in theory be quite efficient if more difficult to implement (because making your propellant go faster makes for more efficiency, for a rocket making the propellant hotter makes it faster, and using a substance to cool the reactor and then using that substance as propellant dilutes the resulting heat more than what you would get from the fuel directly. Usually higher efficiency in this manner means less thrust though). For example, there is a concept called a fission fragment rocket where the individual microscopic particles of fuel that react break off when they do so, and then are channeled together into an exhaust beam. Or in a less elegant and more sheer brute force approach, there is a concept (probably one that won't ever get really built I imagine) wherein you don't bother with a normal rocket nozzle or anything and instead put a strong and thick plate behind your ship, attached by a set of very heavy duty shock absorbers, and then continually detonate a sequence of nuclear bombs behind it to push the ship along.