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?

Question

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

Answer 1

it is because you are thinking of it from two different sides of a curve, which has a minimum and thus a switching point. Fusion of light atoms up to Iron release energy, as their entropy is decreased more than their enthalpy is increased. However after iron it takes energy to create atoms, and they prefer to fall apart (again back to iron) and so fission starts releasing energy for these heavier atoms after Iron. Thus in your theoretical closed system, it would not be perpetual motion, it would go towards iron from whichever side you started and then fizzle out. Now none of this takes into account the feasibility of such reactions, as some of them take incredibly long times to occur.