Friday, April 29, 2016

energy - Fusion vs. Fission


I understand why fission generates large amounts of energy when the nucleus is split, but then why does fusion generate such large amounts of energy. If fission releases energy when some mass is lost as energy, then shouldn't the fusion process absorb energy to fuse nuclei together?


I also am curious as to where the energy released from fusion comes from, while fission releases some of the energy of the strong nuclear force.



Answer



Fission is exothermic only for heavy elements, while fusion is exothermic only for light elements. Intermediate nuclei, in the iron/nickel range, are the most tightly bound, and so you generally release energy moving in that direction.



Fusing stable elements into uranium would consume energy, as would trying to break helium into hydrogen.


For a more thorough background, see for instance this post.


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