Eli5:How does this critical mass trigger a rapid and uncontrolled explosive chain reaction.?
In: Engineering
Critical mass is a bit of a misleading term, really its critical density.
Criticality is when you have bunch of an atom of a isotope that have the property of undergoing fission when hit by neutrons and also release additional neutrons in that fission reaction.
When these atoms undergo fission these neutrons randomly gets sent out with high energy, and they have a small chance of hitting nearby atoms. You have to understand that neutrons are neutral, so they just randomly sent out and, and atoms are very lightly packed, an atoms core is very very small compared to like how tight atoms are connected, it’s like 0.0001% or something of the volume. So a neutron hitting an atom is kinda low chance, unless if you have a ton of atoms.
That’s where criticality comes in, if the newtons released by the fission reaction on average hit at least 1 other atom, you have a chain reaction of fission which is what criticality is.
If you have a lump of plutonium under normal pressure, then you can calculate that at some total mass this average of neutrons causing a new fission reaction overcomes 1 and the reaction can continue forever.
There is something called supercriticality, and that’s basically just if you hit above 1 neutron reaction, the higher you are above 1, the faster and better your chain reaction is.
How do you achieve criticality or super criticality?
There’s 2 main methods, which were also used on japan;
1. Say the critical mass is 10kg, you could pack 2 projectiles of 6-9kg in your bomb, and shoot one at the other, which causes you to hit that criticality. This is a “gun type” bomb
2. take 8-9 kg and pack it in some bomb, then explode a bomb outside of it to massively increase the pressure, this causes the critical mass to decrease and you reach criticality. This is the most common type of nuclear bomb called a implosion nuclear bomb
Another thing to consider is that these bombs usually have some sort of neutron generator, typically this is some super radioactive material that sends out a ton of neutrons, so when you undergo criticality you have a bunch of neutrons that exponentially becomes greater and greater in number releasing more and more energy.
Reddit comments will only go so far to explain this topic. So I highly recommend the following video/ YouTube channel [here](https://youtu.be/LduH7613QXw?si=FG6DW_hcHf10FEG5)
When a fissile isotope spotaneously decays, one or more neutrons are released. Two things can happen to a neutron, it can escape the piece of material and do nothing, or it can be absorbed by another atom of fissile material and trigger that one to undergo fission too, producing more neutrons.
Overall, if more neutrons escape than are produced, the number of neutrons in the material decays exponentially. Contrary, if neutrons can’t escape material fast enough, then more and more are produced in exponential growth, which, if unregulated, is a runaway reaction that results in an explosion.
If this neutron economy is negative or positive depends on many factors. Type of material, amount of it, density of it, shape of it, what other materials surround it and are mixed in etc. But critical mass is meant a spherical mass of a pure element in air, sitting on a table or whatnot. How big would a sphere of plutonium have to be to go boom all on its own. Not very big, about 100mm diameter sphere will do.
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