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 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).
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