This paragraph was responsible for the confusion I had and still have:
"The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons.. The mass of a nucleus is less than the sum total of the individual masses of the protons and neutrons. Work (energy) is required to bring charged protons together against their electric repulsion. This energy is stored when the protons and neutrons are bound together by the nuclear force to form a nucleusThe difference in masses is known as the mass defect, which can be expressed as an energy equivalent. Energ…