Very cool, I didn't know that. So do we call those steam engines or nuclear motors?

Just because an element is part of a molecule doesn't make it a part of something else. Air is mostly nitrogen, not oxygen. You can't breathe pure water vapor, it's not air. Internal combustion engines cannot burn water vapor. If you can figure out how to burn water you'll be a billionaire and save the planet.

That's because the nuclear part only generates the heat, it doesn't provide the motive force. A steam turbine is the motor but we don't call a steam turbine an engine either even though we could. Nuclear power is used to generate electricity which in turn powers electric motors. That's why we don't call them nuclear motors or engines. We do have some nuclear detonation propulsion engines though but they have a whole host of other problems.

An engine is a specific type of motor but not all motors are engines. Engines are thermal devices -- they convert heat energy into motion. You could build a steam engine that doesn't require air but it would still be an engine because it uses the heat expansion of water to create a motive force.