And if splitting atom releases energy, why haven’t these energy break from their atom themselves? Isn’t that means the force that bind the atoms are bigger than the energy released?
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You need a large enough mass to go critical, and you also need it to achieve going super-critical in an extremely short amount of time, that’s why we use explosives to trigger them, if it doesn’t go critical fast enough then it’s closer to a reactor
That’s why even though the demon core was part of an atomic bomb core, when it went critical, it didn’t explode
A single fission reaction does not release a lot of energy, nuclear bombs rely on an exponentially growing runaway chain reaction of fissions. To achieve that runaway chain reaction a so called “critical mass” of fissile material is required.
Achieving super critical mass is not possible without refinement and concentration of the natural ore.
There was a natural nuclear reactor in Africa a couple billion years ago. Ground water collected in a naturally occurring uranium deposit and acted as a moderator allowing a nuclear reaction to occur. The heat from the reaction would boil the water away causing the reaction to stop until more water seeped into the deposit. This continued for (probably) a few hundred thousand years.
It wouldn’t happen today because the uranium isotope that’s capable of sustaining a nuclear reaction (U-235) decays more quickly than the isotope that’s not, so the naturally occurring uranium that’s around today has to be enriched before it can be used to sustain a nuclear reaction.
We do see energy getting released from natural ores which contain uranium. But the amount of energy being released from natural processes is very low. The difference between the fission that takes place in ore and in processed uranium is that the processed uranium produce a runaway reaction. When uranium undergoes fission it produce three neutrons which will cause fission in any other uranium atoms they hit. But in natural ore the amount of uranium is so low that the neutrons will most likely hit other atoms instead. So the reaction stops with one atom, or in some rare cases two. In nuclear reactors we make sure that the conditions are just right for one of the neutrons to hit another uranium atom on average. This means the reaction keeps going and release more and more energy.
Our universe seeks stable energy states. Think of it like a jagged mountain. If you fall off the mountain you’re not going to tumble all the way down to the base. You’re just going to roll to the next ridge or shelf. The same happens with nuclear decay. Once the radioactive atom has released its extra particles it becomes more stable and hard to break apart. That happens again and again, each atom in the “decay chain” becoming more stable than the last.
So the atom doesn’t just blow apart when it decays. It releases just the right amount of radiation to reach it’s next most stable possiblity, no more no less.
A single fission reaction will release some energy, but not enough for any sort of explosion. For a nuclear bomb we want to have a lot of fission reactions in a short period of time.
One way of causing a fission reaction is to take the fissile material and hit it with a neutron. Now the handy thing here is that some fission reactions also release neutrons, and so you can try and get those neutrons to cause even more fission reactions. This is called a chain reaction. When the conditions are right for the chain reaction to continue happening without us having to add more neutrons, we say that it has reached criticality.
Criticality is affected by multiple factors, such as what the fissile material is, how much of it there is, how pure it is, what it’s surroundings are, etc. One thing we do to reach criticality is surround the fissile material with neutron reflectors, these are materials which reflect neutrons back towards the fissile material so as to increase the chance of them hitting the fissile material to cause a reaction. Another thing we do is refine the ore to get the specific elements that are fissile in a high enough concentration.
Without this refining and use of neutron reflectors, there isn’t anything to cause the ore to reach criticality.
With nuclear weapons (as opposed to nuclear reactors), you also want it to happen suddenly, so you need to start off with something that isn’t critical and suddenly make it far beyond critical. Taking the two atomic bombs of WWII as an example, one achieved this by taking two sub-critical masses and firing them into each other, while the other took a sub-critical mass and changed the criticality by increasing the pressure using explosives to create an implosion.
Iron is the magic element. Everything below Iron releases more energy than it takes to fuse the atoms. Everything above Iron releases more energy than it takes to split the atom.
We don’t see random nuclear explosions because a nuclear explosion is an incredibly fast *chain* reaction. Splitting an atom releases (amongst other things) fast moving neutrons. If any of those neutrons hit an atom they will impart their energy to it and if it’s sufficiently unstable then it, too, will fall apart in a burst of energy and neutrons. And if any of those neutrons hits an atom they will impart their energy to it and… well, you get the picture.
The problem is neutrons are smaller than an atom and have no charge so they are not attracted by atoms. Solid matter is mostly empty space between the atoms, so the neutrons can go a fair distance before they hit anything.
So make an nuclear bomb, the first thing we needed was a *lot* of “sufficiently unstable” material. Not all uranium (or whatever the bomb you’re talking about happens to be made of) is created equal, there are different isotopes — that is, versions of the element which all have the same number of protons but have different numbers of neutrons. Some isotopes are more stable than others.
If we used the slightly more stable version of uranium the bomb wouldn’t work. We couldn’t get the chain reaction. So we had to process a *lot* of uranium ore to extract the small amounts of extremely unstable uranium present in it.
But we’re done yet. Now we need to get a “critical mass” of uranium at the same place all at once. If there’s not enough of that highly unstable uranium smooshed together as densely as possible enough of the neutrons would fly through the uranium without hitting enough other uranium to create a sustained chain reaction.
We solved this problem by, basically, creating two masses that were *almost* a critical mass and using precisely timed explosions with conventional explosives to drive those two masses together into one where there’s enough stuff in a small enough space to trigger that chain reaction.
So why haven’t they broken themselves? They do. All the time. Radioactive material is slightly warmer than ambient from the energy released. And, well, that energy is also the radiation in “radioactive.” It’s just really, really slow, at least when compared to an explosion because they’re spontaneously falling apart rather than being driven apart in a chain reaction. When looking at radioactive elements a key piece of information presented is “half-life.” This is the amount of time it takes for roughly half the material to fall apart. A half-life of 12 years means that if you have a pound of it then in 12 years you will have half a pound of it and half a pound of whatever it decays into.
It’s like the difference between lighting a candle and lighting a firecracker. The candle will output more total energy, but the firecracker will be much more abrupt about the energy it’s releasing.
Radioactivity causes explosions only when it reaches a runaway chain reaction. One atom breaks apart and spits out two neutrons, which in turn break apart two other atoms, which split four atoms, which split eight, and so on until you have a runaway explosion.
Those neutrons have to actually hit other sufficiently volatile atoms. In an ore deposit, the “dangerous” isotopes are covered in less reactive ones and just plain rock, so most of the neutrons just get absorbed and the reaction never picks up speed.
We get bombs only when we carefully concentrate only these specific isotopes and in sufficient amounts. Just look at the [demon core](https://en.wikipedia.org/wiki/Demon_core): the two halves of it are safe to handle separately, and the reaction goes supercritical only when the halves are put together, at which point it becomes immediately lethal.
(As a side note: There is evidence of natural “nuclear reactors” where a limited amount of reactivity can occur naturally in radioactive ore veins, but that’s at a very low level and definitely not something that’s comparable to an explosion.)
Uranium atoms, of certain isotopes, decay naturally, at random. When they do, the throw off particles that, if they hit other uranium atoms, cause them to decay approximately immediately. There are enough of these particles thrown off that, if there are other uranium atoms nearby for them to hit, the particles thrown off when *those* atoms decay can hit more atoms, and so on in a runaway chain reaction. That’s what happens in a bomb.
But there aren’t a lot of susceptible atoms out in the world, especially in naturally-occurring ores. And there’s usually a bunch of other crap mixed in, which can soak up the decay particles. If a hundred atoms decay, but the particles they throw off only trigger ninety others to do likewise, that chain reaction will fade away in a couple dozen steps. That’s why, for both power generation and weapons, you need (more-or-less) pure uranium, *and* for the appropriate isotopes to be extracted and concentrated together.
Uranium 235 is what makes a self-sustaining nuclear reaction. The most common isotope of uranium is 238, which doesn’t do this. There is a uranium deposit in Africa that is undergoing sustained fission naturally, but only enough to release some extra heat, not enough to cause an explosion. Normal radioactive decay also releases some heat.
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