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

Fusion of small atoms releases energy but less and less as the atoms get bigger.

After a certain point the start requiring more energy to fuse than they release and even larger atoms require even more energy to force them together.

Fission is the same in reverse. Large atoms breaking apart releases energy, but less and less energy is released with smaller atoms until eventually it start requiring energy to break them.

That dividing line is iron.

All atoms above hydrogen were created in stars.

Small atoms fusing together are the stars fuel. But only up to iron. Beyond iron, the fusing is taking more energy than it is releasing and the star is out of fuel.

Que a star’s collapse and then super nova. In the insane explosion that is a nova, the energy to fuse to all the elements higher than iron is readily available.

So all atoms smaller than iron were created in a stars life, all atoms larger than iron were created in a stars death.