What makes Uranium-235 ideal for nuclear fission? Why not use another element all together?
In: Physics
So there are some explanations here of what nuclear fission is, and some discussions of other elements that also can do it. But you might ask, why uranium-235, and not the much more common uranium-238? Why plutonium-239, and not plutonium-240, or plutonium-238? In other words, why are there only a small number of types of elements (isotopes) that work for this?
The answer gets very technical very quickly, but we can generalize it. If you are looking for an element that is _fissile_ — that can sustain a chain reaction — for various nuclear physicsy reasons they all have:
* very heavy nuclei (like the element uranium and above, which have at least 92 protons)
* they need an _odd_ number of neutrons (uranium-235 is 92 protons and 143 neutrons, whereas uranium-238 is 92 protons and 146 neutrons)
* and to be practical, it needs to be something you can produce in relatively high amounts
Why do you need these things? The heaviness requirement is because the nucleus gets sort of sloshed around. It needs to be _just_ on the edge of stability, so that a little jostling in one direction or the other makes it stretch ([fairly literally](https://www.kullabs.com/uploads/liquid_drop_model.jpg)) to the point where it is only stable as two separate pieces. Uranium is not an absolute limit for this, but it’s right on the edge of near-stability (which is why it is still around naturally after billions of years — it has a very long half-life).
As for the odd number of neutrons requirement, the issue is complicated and gets into how nuclei work. They are sort of a balance between protons and neutrons, though it is not a 1-to-1 balancing. Nuclei with odd numbers of neutrons tend to be more willing to accept a low-energy neutron, like the kinds of neutrons that fission reactions release. Whereas isotopes with even numbered nuclei, like uranium-238, only undergo fission with high energy neutrons, higher than those released by the fission process itself.
This is not very ELI5, but I’ve tried to simplify it a lot, and I have confidence you (and others) who are not literal 5-year-olds can wrap your head around it. The short answer is, there are many isotopes which can be used for fission reactions according to the above criteria, but only a few of them are practical to produce (uranium-235, uranium-233, and plutonium-239, more or less — there are a couple others, but they are much harder to produce than these three).
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