[fennecfoxen]

It's wrong to think of ditching oxidizer and keeping fuel. They both contribute as components of the rocket's energy source and its working mass. If you get rid of the chemical reaction, the "fuel" is not fuel any more, and you should replace your concept of it as such with a narrower concept of "working mass."

The questions here include:

- whether you can get more efficient propulsion overall when the heat comes from a reactor instead of from a reaction in the fuel

- whether switching from a fuel/oxidizer energy-source/mass combo to another working-mass-only substance might net you other engineering advantages (storage concerns, perhaps? though if you're switching to hydrogen like one often considers for nuclear rocket working mass, you will have plenty of storage problems)

- what sort of engineering disadvantages the nuclear reactor component brings (added mass, heat dispersal issues, etc)