Fusion vs. Fission

Well, the answer to this puzzle has to to with the structure of nuclei. (This was an obvious statement, wasn't it?). To see this, let us consider some properties of nuclei. For definiteness, we'll look at helium.

• The most abundant isotope of helium is composed two neutrons and two protons bound into a nucleus by the strong (nuclear) force. Why aren't there helium nuclei composed of two protons? Well, because in such a nucleus, in addition to attraction due to the strong force, the two protons would also repel each other due to the electrical force. The electrical force is so strong (at the close separations found in nuclei), that the nucleus would be quite unstable and would rapidly break apart even if you could produce one in the laboratory.

  In a sense, the purpose of neutrons in a nucleus are to act as electrical buffers between the protons which serves to weaken the electrical repulsion between the protons!

  Based on the above, we note that certain nuclei may be more tightly bound together than other nuclei. That is, some nuclei will require less energy to hold themselves together than other nuclei depending upon the make-up of the nuclei. For example, suppose you fuse A and B to make C. If C is easier to hold together than A and B, energy will be released in the process. If C is harder to hold together than A and B, then the fusion requires an inpur of energy to happen.

  For the latter case, if you reverse the process and think of taking C and breaking it into A and B, energy woudl be released!

• The structure of a nucleus determines whether the fusion process or the fission process releases energy. Unfortunately (or not), the detailed structure of a nucleus must be determined using Quantum Mechanics, but, in any event, such things are only details which can be done.

  It turns out that fusion yields energy for low mass elements while fission releases energy for high mass elements. The break between the two regimes falls at the element iron (Fe) (26 protons in its nucleus). Note that this says that stars can only generate energy via fusion up to elements as massive as iron.