See e.g. Project Orion: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

It’s still not a propellant, and use of nuclear explosives in the atmosphere or near earth orbit is already covered by other treaties.

There’s nuclear thermal rocket engines that have been explored, but those don’t actually use the nuclear fuel as a propellant either. They use hydrogen. https://en.wikipedia.org/wiki/NERVA

NASA was even beginning to study gas–core nuclear engines, where the nuclear fuel is a superheated gas or plasma. Some open–cycle designs where proposed, but NASA choose to study closed–cycle engines such as the nuclear lightbulb engine instead because they don’t leak fissile fuel into the propellant stream. Leaving aside the safety problems, it is uneconomical to lift a fuel into space only to have it escape the rocket unused.

It's technically a propellant - the reaction mass is the spent fragments of each explosion.