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

TLDR: Fusion makes energy with lighter elements, and Fission makes energy with heavier elements

Fusion of light elements like Hydrogen and Helium releases energy but to fuse anything heavier than Iron you need to add more energy to the reaction than it produces.

While Fission uses heavier elements like Uranium that are inherently unstable so they break apart and release energy. That’s what makes them radioactive. Fission of lighter (and therefore more stable) elements would require more energy input than you would get output.

So to make power you can fuse lighter elements, or fission heavy elements.