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?
TIA
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Atoms have varying degrees of stability.
U-235, U-233, and Plutonium-239 are *far* more unstable than a generic iron or aluminum atom.
Due to this, one can easily start a fission chain reaction with these fuels with a source of neutrons.
You need an element that has a lot of neutrons that will be released at high energies in a chain reaction, you want to maximize yield per kg, and you also need it to not… you know… kill you or blow up in the meantime.
Uranium-235 and Plutonium-239 are the elements which fit that bill, they’re stable until you put a supercritical mass together (usually explosively), but they’re also unstable enough to undergo a chain reaction under the right circumstances. Aluminum or Iron is so stable that breaking it apart would consume more energy than it releases, you couldn’t have a self-sustaining reaction.
Edit: Oh the opposite is true for fusion by the way, it’s MUCH easier to fuse lighter elements than heavier ones. You also can only get a net gain of energy through fusing elements up to Iron, after that it’s all downhill. In a “Hydrogen bomb” or boosted fission bomb, isotopes of Hydrogen are fused using the energy of the primary, aka the fission bomb.
It has nothing to do with E=mc^2. Iron is the most stable atomic nucleus because reasons. Elements lighter than iron release energy during fusion, and elements heavier than iron release energy during fission. Uranium is used because it’s far enough away from iron to release a lot of energy during fission, but also close enough to iron to be naturally generated. Uranium-235 also has the somewhat unique quality that each fission event will, on average, produce more than one fission event. This means that if you get enough U-235 together and just one of them fissions, you get a chain reaction of fission reactions that we call a nuclear bomb.
Plutonium-239 is also used in nuclear bombs. Plutonium doesn’t naturally occur on Earth because its half-life is too short. It has to be made here by human processes. Luckily, the aforementioned uranium bomb is a perfect source of Pu-239. Natural uranium ore is roughly 99.3% Uranium-238 and 0.7% Uranium-235. In a nuclear bomb, both types are used in a roughly 1-to-5 ratio. When one of those U-238 atoms is struck by a neutron from a U-235 fission event, it absorbs it and turns into U-239, which quickly beta-decays into Neptunium-239, which beta-decays again into Plutonium-239.
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.
You can break all atoms apart but not all release the same amount of energy in the process.
On the light end of the periodic table you have small atoms like hydrogen and helium.
At that point fusing two small atoms together actually gets you energy and breaking that larger atom apart takes energy.
At the the other end of the periodic table where the really heavy elements live you get the opposite: breaking big atoms into smaller ones gets you energy and fusing them together takes energy.
This is why you can have uranium or plutonium is fission bombs and hydrogen in fusion bombs. Both splitting atoms apart and fusing atoms together can release energy. fusion works for light atoms and fission for heavy ones.
Iron is actually the middle ground. if you keep fusing lighter elements together iron it where that process will stop working without a source of outside energy. Above Iron fission gets you energy and below it fusion works.
Of course you can just use any heavy element for a bomb. You need something that will keep the process going by itself. something that when hit with a neutron will release energy and more neutrons so you get nice chain reaction. Only a few isotopes work well for that. Other are to stable or too instable.
fission bombs are by their nature radioactive because the radioactiveness is what allows them to explode.
If you want to minimize radioactive fallout, you use either normal chemical bombs (or just kinetic if you want to go all sc-fi) or you use a fusion bomb. Of course the only way we have to set of a fusion reaction like that is with a fission bomb.
>I would think that neutrons can break up any nucleus apart.
You would be mistaken. The idea that a neutron blasts into an atom like a bullet is not quite right. Instead, what really happens is that the nucleus absorbs the neutron, so that it binds to the nucleus. Is the neutron is going too fast, it’ll bounce off and won’t really add its energy to the nucleus to destabilize it. If the neutron is too slow it won’t be able to overcome the forces keeping it separate and will again just bounce off and do nothing.
Most isotopes of most elements don’t become unstable when they absorb a nucleus like this. Or at least, they don’t become unstable enough to *immediately* decay. Of the ones that do, not many of them will spit out a neutron in the process of decaying. And of *those* that do, not many are stable enough to be safe and stick around long enough for people to actually use them as fuel. That makes for a pretty short list of candidates.
They don’t just uranium and plutonium, either. They have to use very specific isotopes of it. That’s what the “enriched” part means – they have to increase the amount of the isotopes that they need to a certain percentage of the chunk of fuel.
The farther you go in the number of protons from iron bigger or smaller, the more it wants to be iron. If you go toward the end of the scale (uranium, plutonium, larger) or the other end (hydrogen,helium,) it’s easiest to coax it into becoming more like iron. This releases energy. When stars run out of fuel, even if it’s so large it squeezes anything into iron, they explode and create larger particles than iron, but the reason they exploded is cuz the hydrogen and helium and lithium and whatever else turned into a puck of iron that is too stable and it all falls apart.
A lot of wrong or incomplete answers here. It has very little to do with the stability or radioactivity of an element/isotope.
The answer is because bombarding Uranium 235 or Plutonium 239 with neutrons causes those atoms to split apart and in the process they release more than 1 neutron themselves. These additional neutrons then hit other U235 or P239 atoms, which then creates more neutrons causing a chain reaction. The self sustaining chain reaction is what is important, not the spontaneous radioactivity or stability.
U235 and P239 are not even that radioactive, and they are not the most radioactive isotopes of Uranium and Plutonium.
Elements like Radium are orders of magnitude more radioactive than U235 and P239, but do not have the neutron releasing properties to make usable fissile material for atom bombs or atomic power reactors.
Some atoms are less stable! The bigger the atom, the harder it is (generally) to hold all those neutrons and protons together, meaning it won’t take as much energy to blast them apart.
Simplest reason is two part.
1 st: is the uranium and plutonium naturally give off neutrons as they radiative decay. Granted they aren’t the only ones
2 nd: The waste from the nuclear power plants is closer to the enriching level needed for nuclear weapons. So it gave the government a jump start in building weapons.
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