I am doing a high school research project on nuclear fusion and i just wanted to double check if my understanding of the role of binding energy in fusion reactions was correct. So we add energy in the form of heat to a tritium atom of mass "3" approximately and deuterium of mass "2" , this is the energy required to overcome the strong nuclear force and break apart the atom into its nucleons and that energy is converted in to mass. So the nucleons have greater mass but less energy than the atom. Then we somehow get the nucleons to combine, i haven't researched how we do this yet, to form Helium of mass "4". The mass of the Helium "4" is smaller than the combined mass of our two starting products which have a mass of "5" This mass difference is the energy which is released during fusion and which we can harness. This is my current understanding but i have a feeling something is wrong? Because then how does fusion release energy when two deuteriums are used as the starting product? Another question i wanted to ask is if the mass of atoms which we have calculated using the semi-empirical mass formula is an approximation how do we know the mass of atoms is not a little bit larger and the same as the mass of the individual nucleons?