The binding energy for nucleon in a light atom (like deuterium) is lower than the binding energy for nucleon in a heavier, more stable element like iron. Suppose you remove the neutron from the light atom, and allow it to conjoin (by neutron capture) with an element that had an atomic number near that of iron. Since the nucleon goes from a state of low binding energy to high binding energy, would this reaction be exothermic? I think you see where I'm going with this. Any comments?