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.

​

Edit: Thank you all for explanations! Finally, it makes sense to me.

In: Physics

Answer 1

The simple answer is that you get energy from splitting big atoms and you get energy from fusing small atoms.

If you keep fusing atoms together, you reach a point where fusing them takes away energy rather than releasing it, and the reverse if you’re splitting them. Whichever direction you go you reach iron and then you can’t get any more energy.

(In practice really only fusion with very small atoms and fission with very big atoms is practical. Although apparently if you wait long enough everything in the universe will end up as iron.)