why splitting uranium releases energy but we haven’t see any stray (random) nuclear explosion in natural ore deposits?


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

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.

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.

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.

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.