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

Some fusion and fission reactions are going to absorb energy, but the mass of the specific atoms uses in these processes leads to a reaction that releases energy rather than absorbing it. For fusion in particular, energy is released when atoms smaller than iron-56 and nickel-62 are fused.

Think of it this way:

Fusion – Small mass releases energy
Fission – Large mass releases energy