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?

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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
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To my understanding, only fission of very large nuclei, and fusion of very small nuclei, are exothermic.

Trying to fuse very large nuclei will result in a net consumption of energy.

Both fission and fusion can produce energy, *but* only in certain elements.

Fusion of elements lighter than iron is a net energy positive process, but fusion of elements from iron on up is a net energy *negative* process (hence why very large stars stars catastrophically implode once they start fusing iron in their cores).

In comparison, fission is only net energy positive for elements heavier than iron.

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

it is because you are thinking of it from two different sides of a curve, which has a minimum and thus a switching point. Fusion of light atoms up to Iron release energy, as their entropy is decreased more than their enthalpy is increased. However after iron it takes energy to create atoms, and they prefer to fall apart (again back to iron) and so fission starts releasing energy for these heavier atoms after Iron. Thus in your theoretical closed system, it would not be perpetual motion, it would go towards iron from whichever side you started and then fizzle out. Now none of this takes into account the feasibility of such reactions, as some of them take incredibly long times to occur.

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.)

The atoms which give you a net gain from fusion give you a net loss from fission. And atoms that give you a net gain from fission would give you a net loss from fusion.

Things are a mite bit complicated but the point is, the best case scenario for your hypothetical system is a that the energy gain/loss cancel out.

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.

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.

Both fission and fusion are non-thermodynamic processes. They convert mass into energy, something not allowed by other sorts of chemical reactions. While fusion makes bigger atoms, those atoms have less total mass than the particles from which they were made.

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.)

Fusion energy creates larger atoms but not large enough to use for fission energy.

Fission energy creates smaller atoms but not small enough to use for fusion energy.

Picture a lake that embodies energy equilibrium. Anything heavier above the lake falls, and anything light below the water level raises.
Opposite directions, they both produce movement.