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

If you were trying to use both processes on the same elements, you’d be correct — something that releases net energy through fission isn’t going to release net energy through fusion or vice versa. But fusion will often release net energy from light elements, and fission will often release net energy from heavy elements. So you’re working with different materials, in essence.

It’s sort of like having a hollow plastic ball. If you roll it from the top of a hill, you’ll be releasing energy. If you let it go at the bottom of a pond, it’ll bob up to the surface, releasing energy. But you can’t do both at the same time — a ball at the top of a hill is not going to end up at the bottom of the pond, and a ball at the bottom of the pond will not end up on top of the hill. (I’m not sure that analogy makes a whole lot of sense, but maybe you get the picture — the “starting place” from a potential energy standpoint of the fusion candidates is very different than the “starting place” of the fission candidates.)