system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can both nuclear fusion and nuclear fission create energy? Shouldn't one of this action create and another consume energy according to thermodynamics laws?

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.
gpt
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.