I heard recently a large hurdle to figuring out fusion energy was overcome and while there are still other challenges to face it’s possible that we could one day see fusion reactors.
How is energy released through both fusion and fission? Would it be theoretically possible to split heavier elements into lighter ones and then fuse them back into heavier ones to be split once more?
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Fusing small elements produces energy. Fusing heavier elements requires energy but splitting them apart releases energy.
There is a middle point around iron.
So for fusion you have to use lighter elements; they’re easier to fuse so you get more bang for your buck. On the other hand for fission you have to use heavier elements. But when you fuse the lighter elements they don’t become the same heavy elements you would use for fission. To break that element back into the originals you’d have to put in at least the same amount of energy you got out.
So imagine a middle ground (which happens to be around Iron/Nickel) below which you get energy by fusion and above which you get energy by fission. The problem here is that neither of those actions result in an element that crosses that middle ground.
So say you fuse two hydrogens? You get helium + energy. Helium is still on the lighter side of iron so you can’t use that for fission without giving up energy. And the lighter the atom, the more energy you get, so continuing to fuse the output gives you diminishing returns until you get to the point where it would take energy to fuse to an atom above iron and worth using for fission.
So, the answer is that every element secretly “wants” to be Nickel-62.
Heavier elements are more unstable, and lighter elements are more unstable.
This means you can take *most* elements that are lighter than Nickel-62 and get energy out by getting them closer to Nickel, i.e. by smashing them together so they get heavier – fusion.
Similarly, if you take a heavier element, you can get rid of some of its mass and get energy out by getting it closer to Nickel – this is fission.
Sadly, this means no infinite energy loop. If you try to fuse elements to *beyond* Nickel-62, you don’t get energy out. Similarly, if you try to divide atoms smaller than Nickel-62, it takes extra energy. The maximum energy you can get out from an element through fusion and fission is the difference between the element’s energy and Nickel-62’s energy.
**Edit:** *Lots of good discussion in the comments about whether Nickel-62 or Iron-56 is more stable. Definitely read through them and Google stuff if you’re interested.*
*For now, suffice it to say that the general idea of my ELI5 is right, and Nickel-62/Iron-56 difference is small enough that for most practical purposes you can ignore the difference entirely.*
Protons repel each other but if you shove them together really hard they stick and release energy, more than it requires to shove them together apparently. If you have dozens of protons shoved together they become less sticky due to their repulsion. They release energy when they finally do fly apart as they naturally want to. They won’t fall apart all the way though. And it took more energy to produce those heavy atoms than we got out of them. But they were produced in super novas. But aren’t renewable.
The fusion of atoms releases a small amount of energy. This is because of the slight difference in the mass of the starting particles and the final atom. The loss of mass is expressed as energy. This is because mass and energy both can neither be created or destroyed. So when the mass is lost, it transforms into energy.
Conversely, fission is the atoms splitting. When something like a neutron slams into an atom it can split. This is for big atoms like uranium. They are often shedding particles through radioactive decay. These particles hit other particles. The split from big atom into smaller atoms releases energy from the broken bonds of the original nucleus.
Theoretically it would be possible to take a bunch of hydrogen, fuse and fuse and fuse up to something like uranium, and then split it back down and repeat the process. However, the problem is the middle elements. Light elements like hydrogen and helium are easy (relatively) to fuse. And heavy elements tend to get easier and easier to split. But the middle elements are pretty stable. Silver is hard to fuse. Stars do it. But only when they go supernova. The energy required to get that reaction doesn’t give more than you put in. Then metals like gold are hard to split. They don’t have radioactive decay. They are pretty stable. It takes a lot of energy to split them. Again often more than you get out without making a bomb.
>How is energy released through both fusion and fission?
For a given atom, it isn’t. For any given element, only fusion OR fission releases net energy.
Fusing atoms smaller than iron releases energy, and splitting them requires energy.
For elements bigger than iron it’s the opposite, it’s splitting them that releases energy and fusing them requires energy.
Eg Hydrogen – smaller than iron – releases energy through fusion. Uranium – bigger than iron – releases energy through fission. Fusion of uranium would require more energy than it releases, because just as you suspected it’s impossible for fusion and fission of the same thing to *both* produce energy.
I am amazed that three hours have passed, and 13 posts have been written, but nobody has mentioned the **binding energy curve**.
OP, look at this:
[https://en.wikipedia.org/wiki/Nuclear_binding_energy#/media/File:Binding_energy_curve_-_common_isotopes.svg](https://en.wikipedia.org/wiki/Nuclear_binding_energy#/media/File:Binding_energy_curve_-_common_isotopes.svg)
It is so simple that a five year old could understand it.
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