If my mind serves me correctly, fission is splitting a particle into two, and fusion is bringing those two together, and nuclear fission and fusion produce huge amounts of energy, so why couldn’t we theoretically create a reactor that alternates between those two sometime way in the future?
In: Physics
Yes, you could theoretically make a fission reactor that produced elements which were then re-fused to make fissile fuel. This wouldn’t generate an infinite energy loop because the energy from those fission and fusion reactions is coming from the conversion of a tiny amount of the fuel’s mass into energy.
In other words, each time you split an atom or fuse atoms into a heavier element a small amount of the mass involved in the reaction is lost to produce energy. As you ran your reactor the amount of mass in the fuel would slowly evaporate away until there was no fuel left.
Only certain types of fission create energy. Fusion of hydrogen produces energy and helium. Fission of helium takes energy to produce hydrogen.
Fission works with heavy elements, such as uranium and plutonium.
Fusion works with light elements such as hydrogen.
You can see the energy chart [here](https://openstax.org/resources/6cadef770ccd570f45d1641892c6035fb8edd0ed). You only gain energy if you go up the chart, for example from hydrogen to helium (fusion). If you try to break helium to hydrogen, you lose energy.
Because you always end up with iron.
Fusing smaller elements into larger elements yields more energy than it takes to achieve. Fusion yields less energy as the elements you fuse get bigger, until you produce iron. Fusing anything iron sized or bigger takes more energy than it will yield.
Splitting larger elements into smaller elements yields more energy than it takes to achieve. Fission yields less energy as the elements you fuse get smaller, until you produce iron. Splitting anything iron sized or smaller takes more energy than it will yield.
This is why stars go supernova fractions of a second after they start fusing iron. They produce even more of it in the ensuing collapse. Iron is the most stable atomic element.
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