In a hypothetical isolated system, you could have nuclear fusion reactor and nuclear fission reactor both generating energy. Fusion reactor combining small atoms creating larger ones and fission reactor breaking these large atoms back to smaller atoms, both actions creating energy.
I know that this would be perpetuum mobile, thus it is not possible. I just struggle to understand why.
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Edit: Thank you all for explanations! Finally, it makes sense to me.
In: Physics
Both fission and fusion can produce energy, *but* only in certain elements.
Fusion of elements lighter than iron is a net energy positive process, but fusion of elements from iron on up is a net energy *negative* process (hence why very large stars stars catastrophically implode once they start fusing iron in their cores).
In comparison, fission is only net energy positive for elements heavier than iron.
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