To a very simple approximation, heating air expands it. If you control the shape of the space around it, you can expand it more in one direction than the other.

To a very simple approximation, heating air expands it. If you control the shape of the space around it, you can expand it more in one direction than the other. That generates thrust. This is the idea behind ramjets. Meanwhile, nuclear reactors are hot, and this can be used to heat up air.