Nuclear power provides about 20% of the US electricity supply. To understand how it works we need to understand the nucleus of an atom.
We tend to think of atoms as solid balls linked together with chemical bonds forming molecules. The reality is that the “hard” shell that represents an atom is a representation of the electron that is whizzing around the central placed nucleus. The “solid” ball is a poor representation; instead think of a ping-pong ball. Most of the ball is empty; in the case of an atom the shell(s) (mass of electrons) does not contribute much to the mass, as electrons are very low in mass. The bulk of the weight is in the nucleus where the proton(s) and neutron(s) reside.