I would think that neutrons can break up any nucleus apart. Why not just use aluminium or iron. Is it because of E=mc^2 ? Greater mass equals greater energy? Would a bomb made of another material be less radioactive?
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As you go up the periodic table of elements, atoms become less stable. There is an upper limit to how many protons and neutrons can fit in a nucleus and still have it last for any appreciable amount of time. The nucleus of an atom is made of protons which are all positively charged and want to push each other apart because like charges repel. But the nucleus is also bound by the strong nuclear force which overcomes the electric charge repulsion. As elements get heavier they need more and more neutrons to contribute to the strong nuclear force to keep the nucleus together. So heavier elements after lead are unstable regardless of isotope.
An *isotope* is just a variation of an atom that has the same number of protons but different number of neutrons. For reasons that are way beyond this discussion light elements can also be radioactive and decay but that is through the weak nuclear force which is actually really complicated to explain and not even first year college students in physics really can explain it well.
Some isotopes like Uranium-235 and Plutonium-238 are what are known as *fissile* meaning that they are more likely to undergo nuclear fission in a chain reaction. When these atoms break apart, they don’t just break into smaller pieces, they also release high energy neutrons. If those neutrons strike another one of the fissile atoms it will cause it to split apart and release more high energy neutrons.
If you have enough nuclear material in one place you have what is known as *critical mass*. This is when you have enough fissile atoms together in one place that one splitting apart guarantees that another will split apart. So you get a runaway chain reaction. That’s a bomb.
In a nuclear reactor you don’t want to run away chain reaction, you want to keep the reaction slow and controlled. So so you manage the critical mass of how much uranium is in the reaction, but you also use control rods with a metal like cadmium. Cadmium soaks up the excess neutrons created by a division reaction but does not undergo fission itself. So this takes the neutrons out of the reaction and calms the reaction down.
As the reaction continues the big elements like uranium and plutonium get used up but there is still so much energy you get create all sorts of radioactive lighter elements as they continue to break down. But there is a theoretical limit and that’s iron. Once the products split to iron the reaction no longer creates excess energy by splitting atoms; in order to split iron you have to use up energy. (The reverse is true for nuclear fusion as well, if he feels lighter elements into heavier ones you also create excess energy, but fusing anything heavier than iron takes energy out of the reaction so it stops.)
So this is why lighter elements cannot be used in nuclear reactions, they area very stable and more importantly they don’t break down and release free neutrons when they do decompose in the way that uranium and plutonium freely do.
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