I know that other things are used like hydrogen, but as far as I know, uranium is more common. If all you need is to split an atom to create a nuclear explosion, why use a rare earth metal instead of something more abundant since everything is made of atoms?
In: Physics
To make a nuclear explosion you need an element that, upon absorbing one neutron (a subatomic particle) of any energy level, will split, releasing energy and more neutrons that can continue the reaction. It’s not about just splitting “an” atom, it’s creating a chain reaction so that you can split a trillion trillion atoms in a millisecond or so.
There are only a small number of types of atoms that meet the above criteria (the technical term for this is “fissile”), and of those, only two are “easy” to produce in bulk: uranium-235 (a specific type of uranium that can be separated from normal uranium in special facilities), and plutonium-239 (and artificial element that is made in nuclear reactors). Even these are not “easy” to make by any reasonable standard, but they are a lot easier than the other kind.
So there are real physical constraints on what the options are. Uranium-238, the most common form of uranium, cannot work in a bomb like this, because even though it _can_ be split by neutrons, it can only be split by high-energy neutrons, and the neutrons produced by splitting atoms are not high-enough energy to have a chain reaction in U-238. (This is why you have to separate the U-235 from the U-238.)
Splitting an atom is actually really difficult, uranium (at least the isotopes being used) by nature is radioactive and naturally falls apart. That makes it much easier to forcibly split. Someone else can give you a much more detailed explanation but nuclear explosions have steps leading to the actual split.
For an *atomic* bomb, as you say, you want the nucleus of the atom to split. When it does, since they’re positively charged (because of all the protons), the two pieces will repel each other and (perhaps) crash into other nuclei. So what you want is a kind of nucleus that holds together, but just barely, so that if it’s knocked into it’ll split into 2 more pieces, and so on, creating a fast chain reaction that blows up. A certain kind of Uranium (235) is just right for that.
So there is Ronen’s *fissile rule*, that states, with a few exceptions, for any element with 90 <= Z <= 100, where Z is the proton number, the isotopes that meet 2 x Z – N = 43 +/- 2 are fissile.
A fissionable material is one that can undergo fission when it absorbs a high or low energy neutron, even if the probability of fission is low. A fissile material is one that will undergo fission when it absorbs a low energy neutron with a high probability, and they release thermal neutrons that can be used to sustain subsequent fission events in adjacent nuclei in a chain reaction. Fissile materials are a subset of fissionable materials, they’re more unstable.
The reason why is because odd numbered isotopes have an odd number of neutrons, and due to what is called the “paring effect” described in nuclear binding theory, the nuclei gain quite a bit of energy which can destabilize them. Even numbered isotopes are willing to ignore a neutron, or remain stable if it is absorbed.
That makes for a very exclusive list of isotopes. You have U-233, and U-235, which is more efficient, Pu-239, and Pu-241. Pu-240 can be bred into Pu-241. U-235 occurs naturally, U-233 has to be bred from Th-232.
> I know that other things are used like hydrogen, but as far as I know, uranium is more common.
Alright, from the top – you need a fissile material. So we’ll use either U-235 or Pu. It’s hard to separate Pu-239 from Pu-241, so you tend to end up with a mix of both. It’s a larger atom that doesn’t occur in nature (at least on Earth) and so it’s more unstable, so you can make smaller, more efficient bombs out of it.
Now, you can’t just put this stuff into a lump, it’ll go super-critical on its own and melt itself. So they speak of “configurations” or “geometry”. Modern weapons are derivatives of the Teller-Ulam design, which is a hollow sphere. The metal is thin enough and far away from itself enough to only be critical in this shape. The hollow sphere is then filled with a beryllium chain, which keeps the pit sub-critical for greater shelf-life. They remove the chain when arming the weapon.
Now you surround the thing with conventional high explosives. The explosive can be either fast burning or slow burning. The reason being you want uniform compression, but you can’t achieve uniform ignition. So if you have two shock waves emanating from two different ignition points intersect, you’re going to get a high speed jet, which is bad. By carefully shaping and layering the explosive, you can get the shock waves to meet up evenly, and then come down as efficiently and uniformly as possible.
Now is a good time to explain the tamper. This is a metal sphere, often made of lead, but also sometimes uranium, that takes the brunt of the shock wave. The point is to provide a surface that can even out any irregularities in the shock wave. There is a physical air gap – often made with Styrofoam, between the pit and the tamper. This gives the tamper space to accelerate. As it gets crushed down, it slaps into the pit, providing as even and uniform a compression as possible.
All this compression is to bring our hollow sphere into a very small, dense lump of fissile fuel. You can get the atoms closer together this way than if you just poured metal into a mold and made a lump. The distance between individual atoms shrink, so that the chances of a neutron striking a nuclei is basically assured.
Now is a good time to mention exponential growth. All of this happens in a fraction of a second – the detonation, the compression, the fissile chain reaction. The random nuclear decay of these heavy elements provides enough neutrons to start the chain reaction that converts a lot of the material into fission byproducts and pure energy. Smaller bombs are less efficient. The largest pure-fission weapons were up to 50% efficient, but typical is 25%. Fat Man was 17% efficient, and it was a plutonium bomb with a uranium tamper. Little Boy was a uranium cannon ball fired from a cannon into a uranium plug, all inside the bomb housing. It was 1.7% efficient. The Tsar Bomb, the largest weapon ever detonated, as a shock and awe demonstration, was the most efficient weapon ever detonated, and actually left very little fallout.
This fission weapon can be boosted. A hydrogen generator is just a chemical reaction that makes free hydrogen gas which is injected into the hollow pit. It’s added for its neutrons. It only takes hundreds of atoms to have a pronounced increase in efficiency. They also use flash tubes, a form of primitive particle accelerator, to inject x-rays into the pit at the moment of criticality, to introduce more energy, more instability. That Styrofoam? It flashes into plasma, which is magnetic, and helps with confinement and reflection of neutrons back into the pit. And the outer housing is made of neutron reflectors. Anything that can be done to increase confinement will increase the efficiency and yield of the weapon.
Back to the configuration, we don’t use hollow spheres, we use hollow ovoids. What matters is that it crushed down to a sphere in the end, it doesn’t have to start that way.
Ok, now we get to thermonuclear weapons. First, you need a fission weapon as above. This exists solely as a compression stage, just like how the conventional explosives and tamper did for the pit of the fission weapon.
The second stage is a paint bucket with a hollow rod through the lid to the bottom. The rod is called the spark plug, and it’s made of plutonium. The can is filled with lithium deuteride. This is lithium bound to hydrogen atoms. Here is where you get the hydrogen in hydrogen bomb. Due to the shock wave, and other forces the public record doesn’t really talk about, but we think there’s also electromagnetic and x-ray compression going on, the bucket is compressed. The spark plug goes fissile, the compressed lithium gets showered with neutrons. The lithium and hydrogen fuse, releasing free hydrogen and neutrons. Thermal neutrons. Hence the name thermonuclear. These fast neutrons are moving at 17% the speed of light.
That bucket I described, holding the lithium? It’s made of U-238. It’s not fissile, but it doesn’t have to be – fissionable is enough. The thermal neutrons flash right through them like they’re not even there. It’s plenty of energy to cause them to fission. This is how we get the big bang of fusion bombs.
The Tsar bomb? It was some 26 fusion bombs that surrounded a 3rd stage. Same thing, the same bucket. They used a tamper between the second and 3rd stage. Originally it was going to be made of uranium, but they switched it to lead to increase the probability of a successful test. They say it was 50 MT, but that’s contested, it was likely more. Had the tamper been uranium and the test successful, it was expected the yield would have been doubled.
> why use a rare earth metal instead of something more abundant since everything is made of atoms?
Because not all atoms are created equal, as I discussed early on. Some atoms will let neutrons pass right through them, some will absorb neutrons all day and never split, some will reflect them.
What you might notice is energy is yielded from both fission and fusion. And indeed we are chasing after both cleaner, safer, more efficient fission in nuclear power plants, and the ever elusive promise of fusion power.
How is this? Why don’t we split atoms and fuse them back up again for free energy?
It turns out, smaller atoms need less stuff to be stable than larger atoms. So when you split a big atom, you don’t just neatly cut it in half, you also cast off excess particles and energy that the smaller byproducts don’t need and can’t hold onto.
Likewise, the same is true of small atoms. Larger atoms need fewer parts than the sum of the atoms they were fused from, so the excess is cast off.
There is an intersection, though, where fusing smaller atoms need more than the sum of their ingredients, and splitting atoms takes more energy than their byproducts.
That intersection is iron. It takes more energy to split iron and smaller than you can ever yield from the fission itself, and it takes more energy to fuse iron and bigger than you can yield from the fusion itself. When stars start forming iron in their cores, they have seconds to live before they super nova. This is why. Elements larger than iron are formed in the catestrophic collapse and rebound of the star when it implodes then explodes. More is created if you have something like a neutron star collision, and material gets thrown free into space. That’s where all the heavier elements come from.
Roughly speaking: the heavier an atom you split, the more energy you get by splitting it.
Uranium is the heaviest element that’s abundant *enough* for us to build things out of.
Modern implosion-type nuclear weapons are made from highly enriched Plutonium-239.
Uranium-235 is used for gun-type nuclear weapons because Plutonium was found to have a high probability to “pre-initiate” and not reach full yield.
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