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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I’ll try an actual ELI5 explanation
Imagine you have alot of matches that can spontanously ignite. Now imagine a nailboard of those matches with the ignition side pointing up. In nature, you do have these matches but they are far apart from each other. One can randomly ignite but it’s too far from the others to ignite them aswell. If one ignites, you can see the light and maybe if you put your hand next to it feel the warmth but thats about it. The macthes just aren’t close or concentrated enough. You can messure this small activity and Energy release in natural Uranium Ore
Now imagine if you put all the matches on that nailboard really close together. Now one randomly ignites, which ignites the one next to it, then the one next to it and so on. After a very short amount of time the whole board ignites in a big flame, with lots of light and heat. That’s what a nuclear bomb is. So much reactive Uranium stuffed together that it reaches a “ciritcal mass” and explodes.
There’s a difference between radioactive decay and fission.
Uranium is radioactive – it spontaneously releases an alpha particle ( a helium atom) from itself to reach a more stable configuration. This process is happening all the time. In fact, it’s theorized that heat from radioactive decay is what has kept the core of the earth hot and molten for the past few billion years.
What is highly unnatural is fission. Fission involves actively throwing a neutron at high* speed toward a uranium atom. Since decay of uranium does not produce a neutron, it cannot self-start a fission event.
note: the term ‘high speed’ here is relative to human experience, is somewhere between 2km/s and 14,000km/s
In order for uranium 235 to split it needs an activation, which is typically a neutron.
You don’t have a lot of free floating neutrons just flying around. When uranium 235 splits it releases 2-3 neutrons that can go on to impact other uranium-235 atoms, however the chances of a neutron hitting a nuclei is extremely low, they are very small. For criticality, that meaning a sustained nuclear reaction to happen, you need lots of uranium-235 very closely together so that on average at least 1 of those neutrons from a uranium-235 decay hits another uranium nuclei. That only happens in bombs and nuclear reactors.
There’s also tons of oil and natural gas deposits all over, and they would love to burn and explode to release all that energy, but just like uranium, you need to give it a push to start that reaction.
A single atom splitting only releases a tiny amount of energy. So in uranium ore you have atoms splitting but not enough to do anything other than make it a bit warm and maybe give you cancer if you eat it.
To make an explosion you need a large lump of extremely pure uranium to get enough atoms splitting to actually release that much energy.
Basically, the natural state of elements likes stability. The state of an element being unstable is defined by it’s natural attempts to reach stability, either by shedding subatomic particles or by bonding to something else.
When we make nuclear reactors and nuclear weapons, we are creating our own unstable isotopes of elements. In a really simplified way, what we are doing is finding the most unstable versions of elements from nature, and then bombarding them with sub-atomic particles until they are as unstable as they can be without being volatile.
And then when we talk about starting a reaction, either for reactors or for weapons, we are further bombarding those elements, in an enclosed space to increase the chance of the reaction going critical (up to 100% chance).
None of that happens readily in nature. Even if unstable isotopes form in nature, they are likely more stable than what we use in reactors, and even if there are very, very unstable isotopes, they likely won’t be bombarded with subatomic particles to the point that they would create a reaction, and even if they did create a reaction, they likely wouldn’t have the contained space or sustained particles to maintain the reaction. So in nature, they are rarer.
This is the same misconception that I had for a long time. Splitting a single atom doesn’t release much energy, not enough that you could notice. We do sometimes see this naturally, when concentrations of these elements occur we experience the energy they release as radiation from their natural radioactive decay. This radioactive decay is random, but with enough material you might be able to measure it with instruments or feel it’s energy through heat.
A nuclear explosion happens when there is a self-sustaining chain reaction of trillions of Uranium atoms all at once. Random radioactive decay is too slow, you have to set up the material in such a way that the radioactive particles bump into other atoms and cause them to also release radioactive particles and so on. In order for this to happen you need to have trillions of these atoms all very close to each other.
If by explosion you mean fission, this does occur in natural uranium deposits, however if you were to purify the ore down to just uranium, only 0.7% of that uranium would be uranium 235, which is the primary isotope producing these fissions. At that concentration there’s not enough to sustain a chain reaction. If you were to filter off the U235 by enriching it, you could eventually reach a purity where a fission chain reaction is possible with the help of moderators, and if you go further to like the high 90%s you could reach a point where, if you have enough of it, a runaway chain reaction like in a nuclear bomb is possible.
Individual uranium atoms split (fission) all the time. You can measure this with the right tools. The reactions release a microscopic amount of energy.
The energy released in a nuclear explosion or nuclear reactor is a _chain reaction_ of those fission events. So one atom fissions, and its neutrons cause other atoms to fission, and their neutrons cause other atoms to fissions, and so on. In a reactor this is slow and controlled, in a bomb is very fast.
There are specific conditions that need to be created for these kinds of chain reactions to exist. At the most basic level, you need enough of the right kind of uranium (U-235) near itself, without too much stuff around it that will also absorb the neutrons. If you want an explosive reaction, you need almost entirely U-235 by itself.
There are other things you can do to change the conditions to make these reactions happen — there are many ways to design nuclear reactors. The long and short of it, though, is that uranium as it is today found in nature requires very, very specific conditions to work in a nuclear reactor, because its concentration of U-235 is very low. As others have noted, 1.7 billion years ago, the concentration of U-235 was higher in natural ore, and enough that natural conditions could produce a sort of nuclear reactor. But because of nuclear decay, natural uranium ore today could not produce those conditions in nature. It is possible to use uranium with the same enrichment as natural ore in a reactor, but it requires creating very artificial conditions (like rendering all of that ore into metal, putting it into very specific geometries, having very specific “moderators” that increase the chance of fission).
At no point in the past was it ever possible to have a natural nuclear explosion. The requirements for a nuclear bomb are very specific and very artificial. They can be achieved — a nuclear bomb is just an engineering device for achieving those conditions — but you will not find them in nature.
Nuclear explosions require a lot of energy to start the chain reaction that leads to the huge explosion. In a nuclear bomb, the first thing that happens is a conventional non-nuclear bomb is detonated to compress the uranium. This type of energy release is very unlikely to happen spontaneously in nature, in a uranium deposit.
**tl;dr**
A nuclear explosion requires density. Refined nuclear fuel is denser than what’s found in nature. Outside of outlier examples like the naturally recurring reactor mentioned in another comment, this isn’t common in nature.
**Example 1: Natural Uranium**
Imagine you’re standing next to a big empty swimming pool that has maybe two or three beach balls scattered in it. You throw another beach ball in the pool, not aiming at anything in particular.
It’s probably not all that likely that your ball is going to hit the others right? And even if it did, the ball you threw isn’t likely to hit one of the other ones, nor is it likely that any ball it did hit would hit one of the other ones.
This is kind of how it works with natural uranium. There isn’t a high enough concentration of nuclear “stuff” to run into each other over and over; which is what a chain reaction is; which is what causes a nuclear explosion/meltdown.
Further natural uranium isn’t really pure, meaning that in our pool, there’s other stuff getting in the way of the beach balls. We’ll go with basketballs for our example. So if there are a bunch of basketballs in the pool, there’s even less of a likelihood that the beach balls will run into each other, because there’s stuff in the way.
**Example 2: Refined uranium**
Now put a couple hundred beach balls in that pool. If you throw another ball in there you probably are going to hit one of those other balls; and when you do it’ll probably bounce off and hit another; and those balls will probably hit other ones. Creating a chain reaction.
When we talk about “refining” nuclear material, we’re talking about doing something to make sure there are as few basketballs in the pool as possible; and as many beach balls as possible; making it really really likely that beachballs will run into each other to make energy.
**Meltdown vs Explosion**
Your question was specific to “explosions,” so lets address that one real quick. Basically the difference between a meltdown and an explosion is speed.
In our pool of beach balls, in a meltdown situation the thing that is controlling the likelihood of beach balls running into one another is two things: temperature and density. The density thing is explained in the examples above: more beachballs closer together means it’s likelier they’ll run into each other. For the temperature thing, we use some kind of fluid to keep the beach balls from moving to fast; basically because colder things move slower. So as beach balls move slower, they still hit each other, but less often.
Remove the temperature control and they just hit each other faster and faster overtime and overheat, resulting in a meltdown. But it does take time. This is why Chernobyl is a wasteland and not a crater, because the reactor got hotter and hotter over time, which damages equipment designed to contain it and releases all that nuclear gunk into the environment.
Now to produce a nuclear explosion, you don’t want to just let the reaction slowly build over time, as in the natural reactor example in another comment; you want it to build really really quickly all at once. Like in an instant. To do that you need to increase the density (how close the beach balls in the pool are to one another) really really quickly.
Using the bombs dropped on Hiroshima an Nagasaki; they did this in two ways. Two different fuels (plutonium for one, and Uranium for the other), but the principle is the same. For fat man, they created a ball of material and then wrapped it in explosives that pushed all that nuclear material into a smaller and smaller ball really really quickly; dramatically ramping up how fast and how likely reactions between different bits of nuclear material could occur.
You ever take a little piece of white bread and make a ball to snack on? (I don’t know, I did this when I was 5). It’s kind of the same thing. The white bread starts out all loose and fluffy, but when you shape it in to a ball and push it together it gets much harder, or more “dense;” meaning that all the little bread bits are really close together. Imagine if you could do that and form a perfect ball in an instant that was as hard as possible. That’s how it works.
For Little Boy, they used what’s called a “gun type” weapon; that one worked by basically firing a piece of nuclear material into another one. When one hits the other, density is achieved and boom. Explosion.
In nature (on earth), you don’t have really dense, refined concentrations of nuclear material slamming into one another or becoming instantly concentrated. You sometimes do have deposits that are pure enough to produce energy, which is why the natural reactor example is possible; but the natural explosion idea isn’t all that likely.
**Edit 1. To Respond to part of OPs question I left out:**
>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?
When we say “energy” here, it’s not like an atom is a literal box holding a power force waiting to be let loose. We talking about the energy it could produce if enticed.
For example, think of a block of wood. Do blocks of wood spontaneously burst into flames because they “contain energy?” No, because that energy doesn’t just release itself. However, we can measure how much “energy” a block of wood has because we can measure how much heat it could put off/what that heat could power (like with steam or something).
Now think about Coal. Coal burns hotter than wood; and so has more energy than wood, because the same amount of coal and wood would produce different amounts of heat and power.
Now if you had a single stick or a single lump of coal and tossed a match at it would that be a big deal? Probably not. How about a forest or a coal mine? Now you have a big problem.
Now think about an atom. Extremely tiny, but has a lot more energy than wood or coal. If one atom splits do you want to be near it? Probably not, but it’s no where near anything close to if you had many many atoms really close together.
So think about an atom as a tiny block of wood, that “burns” really really hot and fast. So hot and fast that you don’t want to be anywhere near it if its “burning;” but not so incredibly unpredictable that it’s going to spontaneously combust.
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