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?
In: 508
As for the last question, there is a difference between whether a reaction releases energy and whether an action is *spontaneous*. If you imagine a ball on a table, the ball has the potential to release energy when it falls, and may be a bit unstable. But the ball will not *spontaneously* fall. An intervention is required to get it to the lower energy state.
The universe doesn’t “know” that the ball falling is the better energy state. It knows that, in order to fall, the ball *needs* a bit of energy (in the form of a push) and the ball does not have that energy right now. There is a little energy barrier to get the ball to release energy, so it doesn’t.
[deleted]
Same reason if you spread gunpowder across a table it’ll burn, but if you pack it tight and ignite it it’ll explode.
Density.
Uranium atoms are splitting all the time, but they don’t form chain-reactions very often because they’re not packed tight, and the uranium ore (Pitchblend) isn’t in a very pure form either.
Natural ore deposits are basically radioactive dirt, and they get warm from their reactions and can stay that way for millions of years.
Once you dig up that dirt and strip out the uranium and put it all together, it starts getting a lot hotter (flash-boil water to steam levels of hotter)
In general Uranium isn’t unstable enough to make a bomb though.
For that, you want Plutonium, which is generally found as a byproduct of uranium decay.
As the uranium breaks down due to its own radiation, it tends to form several other materials, most of which are short lived and unstable.
Isolate the Plutonium from that, get loads of it together (a few pounds will do) and you’ll find it’s a hell of a lot more radioactive than uranium, which is what you want for a bomb, because what you basically want is a material which is a house-of-cards waiting to be knocked down all at once.
Squeeze it all together tight enough, and it’ll explode if you have enough of it in one place.
The stray neutrons from its instability have nowhere to go but smack into another unstable atom, which makes more neutrons, and breaks up more atoms, and more and more until the whole thing is coming apart, rapidly releasing huge amounts of energy in the form of heat
In a nuclear bomb, about 5 – 15 percent of the mass will undergo decay like this during the detonation, and that’s a hell of a lot of energy at once.
TLDR, natural uranium deposits aren’t packed tight enough, or unstable enough to explode.
If you have a pool table full of balls that are touching each other and you push one ball then all the others will also move. But if you spread the balls apart, then nothing happens. Same thing with nuclear explosions, the Uranium needs to be pure enough that all the atoms are sitting next to each other when the “push” is made. This level of purity doesn’t happen in nature.
>And if splitting atom releases energy, why haven’t these energy break from their atom themselves?
We see it all the time, it’s called radiation, and there’s radiation all around you in the environment at all times. It’s just a very tiny amount because there’s only a tiny amount of this material.
To make a bomb you need the nuclear reactions happening really fast, before the bomb has a chance to fully blow up. It’s really hard to get that much fissile material to be unreacting in one moment, but then fully reacting in the next, without just blowing itself apart before it has a chance to react very much.
That’s why bombs today are really complicated. They start with a regular bomb that has the nuclear material at the center of the bomb. The outer bomb compresses the fissile material inside, and that compression makes all of the material supercritical at the same time. Add some neutrons to kick it off, and this is what gives us the nuclear explosion.
We actually see evidence of natural uranium reactions in some uranium ore deposits. Like geysers, water would carry just enough material to cause them to briefly react and generate some heat but because you don’t have enough of it in one place, it doesn’t explode.
For the same reason you can make dynamite out of animal fat, feces, and dirt, and yet a pile of such industrial inputs does not spontaneously explode. Uranium has to be enriched, which is to say, the two naturally occuring isotopes have to be separated, and the fissile isotope, Uranium 235, has to be concentrated by getting rid of the non-fissile isotope, Uranium 238, until the concentration is high enough for the chain reaction to sustain itself.
In naturally-occuring Uranium, only about 0.7% of the element is U235, and in order to be used as fuel, it must be enriched to between 3 and 5%. In order to be used as a bomb, it must be refined to a MUCH higher purity, for example, the average enrichment of the Little Boy bomb exploded over Hiroshima was 80% U235. This, of course, also ignores that naturally occuring Uranium isn’t pure elemental Uranium, it’s compounded with other elements to form an ore, such as
[pitchblende](https://en.wikipedia.org/wiki/Uraninite).
So, you dig up the rocks, you subject them to chemical refining to remove the parts of the rocks which aren’t Uranium, and then you subject the pure Uranium to enrichment, which means you’re removing large volumes of the U238 isotope.
Imagine an Atom is a ball on a table.
The ball could fall from the table and release energy, but it might also not, depending on wether it is on the edge or not, is something agitating the ball/the table, etc.
There is energy stored in the ball (because of it’s height), but it might not release that energy on it’s own. It might take some time for the ball to randomly roll to the edge and fall, or be agitated and pushed to the edge by something in the environnement.
.
We could have a bunch of balls on a bunch of tables, wait for some to fall and take that energy, but they don’t naturally fall often enough to be usefull as an energy source.
If we want to use these balls as an energy source, we must find a way to trigger their fall: that way we could have a lot of balls falling in a short amount of time, producing a lot of energy quickly, making it a usefull energy source.
Having to individually push balls off their tables is not feasible or efficient, but what if, when one ball fell, it could bump into other tables, causing their ball to fall, which would go and bump into other tables, etc. This is a chain reaction: if we can arrange the right conditions (having a lot of tables close to each other), we juste have to wait for on ball to naturally fall, to trigger à bunch of other balls, and before you know it we have a lot of balls falling quickly = usefull energy source.
The conditions necessary for a chain reaction to occur are very rare in nature (there is one exemple of a natural nuclear reactor in oklo, Gabon, see other comments), so big nuclear reaction/explosion are man made.
Most of the time you just get a slow, natural and random splitting of atom in the material. Enough to produce some heat and maybe affect your health if you stay close to it, but nowhere close enough to speed and quantity of splitting atoms in a man made chain reaction.
When I was a kid, I thought that I’d you had a sharp enough knife and hit a cutting board with it, that you could split an atom and accidentally cause a nuclear explosion.
It takes (relatively) a lot of uranium in one place to create a nuclear explosion. It’s extremely unlikely for that much to collect in nature in one place.
As for the second question: some elements (those that have bigger nuclei than iron) release energy when their atoms split apart, and some (those that have smaller nuclei than iron) release energy when they fuse (that’s how fusion, or “H-” bombs work).
Even then, for any element that is relatively stable (doesn’t decay very quickly on its own), you need to put some energy in to make fission happen on demand. Like popping a balloon: you need to put in a bit of energy to break the surface, and that releases the rest of the energy that was stored in the tension and pressure and makes the popping sound.
I will take a stab at this.
> ELI5 why splitting uranium releases energy but we haven’t see any stray (random) nuclear explosion in natural ore deposits?
To “explode” you need geometric growth. An atom undergoes fission releasing neutrons, these neutrons trigger fission releasing more neutrons. In the case of Uranium 235 there are approximately 2.5 free neutrons per fission event, for Plutonium 239 there are approximately 3 free neutrons. You can see that after a few generations the number of free neutrons has become rather large.
For this to take place you need enough ‘fissile” material in a single location and this doesn’t exist in nature. If it did, it would have undergone fission long ago. The bare naked sphere critical mass for Uranium 235 is around 52 KG and just 10 KG for Plutonium 239. (There are ways to reduce this number)
~2 billion years ago the ratio of U238 (currently 99.3%) vs U235 (0.7%) was at the same levels we see in enriched reactors today. So as others pointed out already there was at least one known “natural reactor” which used water as a moderator and the U235 underwent fission.
A single atom undergoing fission does not release a whole lot of energy, but again after several generations the number of atoms undergoing fission is substantial.
> 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?
There is a concept called the “fission barrier” which is the amount of energy you need to put into an atom to trigger fission.
Lets keep it simple and say Uranium 235 has energy “10” and a free low speed neutron has an energy of “3”. Further, let’s say the fission barrier of U235 is 12. If the free neutron is captured by the U235 atom its ‘internal energy’ is now 13. This exceeds the barrier and so fission takes place.
In the case of U238 we can say the atom has an energy of “11” and the free neutron has energy “3” and the barrier is 15 (it has more neutrons and so it is more tightly bound). in this case, U238 becomes U239 because the new neutron has not caused the atom to exceed the barrier.
A high speed neutron entering a U238 atom can trigger fission, but the faster the neutron the less likely the interaction. (you can take the neutrons resting energy and its kinetic energy and if this exceeds the barrier fission will take place).
(do note this is simplify and the energy levels are not “3”..)
Fission does take place in nature (via quantum tunnelling to avoid the energy barrier issues). But it is RARE (elements which could do this have already done so).
Latest Answers