In fusion, the mass of the reactants is greater than the mass of the products. Thus, mass is lost in fusion. My thinking was that this mass defect is converted into binding energy that is then used to hold together the products and thus have a nucleus. But then I found out that kinetic energy is released during fusion. How can this be? Wouldn't that mean there's no binding energy.
Look at the binding energy curve per nucleon:
This is what allows for kinetic energy to be released, for nuclei below Fe. If two nuclei with low binding energy per nucleon join and form a nucleus of higher binding energy per nucleon, the difference is released.
Look at this deuterium tritium fusion . He4 nucleons are more tightly bound, so energy can be released.