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I know how nuclear propulsion is used on board ships and subs, but I’m wondering how it would be used in spacecraft.

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Seems it’s got to eject mass in order to make thrust.

They are used to raise a high voltage, which is then used to accelerate an ionized gas out the thrust nozzle. Heating a gas would also provide for a fast moving reaction mass.

Chemical rocket engines are extraordinarily powerful but burn their fuel quickly and inefficiently. Nuclear rocket engines though are efficient but generate less thrust.

A nuclear reactor rocket engine heats a material of one sort or another in order to make it serve as a propulsion mass for a rocket engine exhaust. In contrast, in conventional rocket engines, a chemical reaction between the fuel and an oxidizer generates heat quickly, with the residue then being explosively ejected from the rocket nozzle as propulsion mass. This generates massive thrust quickly but is highly inefficient.

Since nuclear reactors are far more efficient at generating heat than chemical reactions, they can propel a spacecraft faster and farther by heating mass than can be done by a chemical rocket engine. The so-called fuel fraction can therefore be smaller in spacecraft powered in whole or in part by nuclear engines.

Even more efficient nuclear rocket engines are possible by using electromagnets to accelerate an ionized reaction mass to greater velocity than is possible with simple heat transfer. This efficiency though comes at the price of generating smaller thrust, so such engines would be used primarily for slow, long duration space flights.

In current plans, conventional chemical rockets would be used to boost nuclear rocket engine parts to orbit where they would be assembled and used for space flight to the Moon, Mars, and other destinations. In such flights though, chemical rocket engines would still be used where high thrust is needed for maneuvering.