The difference between an nuclear bomb and a hydrogen bomb.
In: Engineering
Fissile isotopes are radioactive variants of some element that decays when it absorbs neutrons, and releases neutrons when they decay. Fissile isotopes can sustain a chain reaction, where one decay can cause an adjacent atom to decay.
If you google it, Wikipedia has a formula used to predict which isotopes are fissile, there’s a lot more than you think. The equation will produce nonsense, in that some isotopes don’t or can’t exist (but that’s math for you), and the equation doesn’t tell you other properties of the fissile material, which are important for use as a weapon.
So, then, to cause a nuclear explosion, you want your fissile fuel to decay as fast and as completely as possible.
Chain linear reactions are super slow. One atom decays, causing another atom to decay, which causes another atom to decay… And how do you start this decay process? In order to decay your fuel *fast*, and if you could induce decay in the first place, then you’d be better off just inducing decay in the whole mass of fuel all at once. But we do depend on chain reactions, and that tells you we can’t induce decay, let alone in an entire mass.
So this is what that equation I mentioned above doesn’t tell you: of the fissile isotopes, *only a few* release two or more neutrons. That means the reactions aren’t linear, they’re exponential. The number of isotopes decaying can double over time, and that’s something that just cannot happen with a linear chain. The fuels of choice are Uranium 235 and Plutonium 241. Plutonium 239 also works, just not as well. We don’t have a process to reliably separate the two isotopes, so plutonium bombs have a mix. U-233 also works, but not as well, though we can separate it out, so our enriched weapons have low counts of that. Uranium 238 IS NOT fissile, in that it can absorb neutrons without decay.
Alright, now we need our fissile mass to undergo chain reactions very fast. The best way we know how to do that is to increase its density. You can do that as we did with Little Boy – this bomb, dropped on Hiroshima, was basically a cannon. The bomb casing was a glorified cannon barrel. The projectile was a 60 pound slug of u-235, and at the end of the barrel was a plug, in the shape of a cone pointing into the cannon, made of another 60 pounds of u-238. The bomb fired the slug into the plug. They slammed into one another, compressing, increasing their density.
Radioactive decay is essentially random. And so we rely on that random decay to start the exponential chain reaction. We’re talking a few tens or hundreds of random decays causing a city-erasing explosion in a fraction of a second. When the fuel is so dense, those random neutrons that went flying away have a greater chance of hitting an adjacent isotope. That’s all it takes.
From there, it’s just a matter of holding in the explosion as long as you can, because fission releases a lot of heat, and so while you’re trying to compress the mass, the heat wants to blow it apart. The longer you can hold it together, the more isotopes you’ll convert, the more efficient the bomb, the bigger the explosion.
Little boy was only a few percentage points efficient. 120 pounds of u-235 for a few kiloton explosion, modern weapons use about 20 pounds of u-235 for up to megaton explosions.
Then you have the implosion style weapons. All modern weapons are implosion style. Plutonium works well for this. There was an initial effort to refine the gun style weapon, because it’s damn simple, but there were inherent limitations. Little Boy had a bigger brother, Big Boy, a gun style weapon for plutonium. The project was cancelled early on because the bombs were 50 feet long or so.
Modern day implosion style weapons use a hollow pit in the shape of an egg or oval, but all descriptions of the original Teller-Ulam device presume a sphere. It doesn’t matter so long as the pit compresses into a sphere in the end. The reason it’s a hollow pit is because the geometry, called the configuration is important. A solid lump of plutonium will go critical and just melt from the heat of sustaining its own uncontrolled chain reactions. It’ll melt down just like a nuclear reactor gone awry. You have to keep the stuff away from itself to keep it stable.
So you surround it in high explosives. But the explosives are layered. Because you can’t ignite every point on the surface uniformly, you’ll generate shockwaves. So you have fast burning explosives detonate domes of slow burning explosives, until the domes all meet up – at which point you have a uniform shockwave, which then goes to fast burning explosives again. The shockwaves hits a tamper, which is a piece of metal between the explosives and the pit. All this thing does is even out the shockwave. The tamper is air-gapped, so it has space to pick up speed and inertia – the pit is hanging inside by wires. The tamper is made of lead, but sometimes the tamper is u-235 to increase yield, but it can make detonation unreliable. They also inject hydrogen atoms into the center of the pit, they use flash tubes – a type of particle accelerator, to inject X-rays, and surround the whole thing with polyethylene foam impregnated with neutron reflecting materials. The foam itself vaporizes into plasma, lending electromagnetic properties to further reflect and compress the pit. The hydrogen and flash tubes are how you can dial-a-yield, by varying their timing, thus varying the weapons efficiency.
All that is atomic bombs and “boosted” atomic bombs. Let’s talk about hydrogen bombs now, or thermonuclear bombs.
To go thermonuclear, you need a second stage. Just as we compressed the first stage with conventional explosives, we’ll compress the second stage with an atomic bomb.
The second stage consists of a u-238 bucket filled with lithium-6 and -7. Notice the bucket is not fissile, but it is still radioactive, heavy, and energetic. Remember when I said we can’t just magically induce atoms to split? Yeah, that’s not true. This is how we do it.
So by compressing the second stage, the lithium undergoes fusion. In the process, the fusion products are more stable atoms than the lithium, and they cast off excess alpha particles – hydrogen! This fusion process is extremely energetic – these hydrogen atoms are just neutrons. **THERMAL** neutrons. When you energize a neutron and flick it off into space, the amount of energy it carries and how fast it moves are categorized. Thermal neutrons are WAY more energetic than those of our previous fission process. They’re moving at 17% the speed of light. They fly right through that u-238 housing like it isn’t even there, causing it to decay instantly. This is where the big bang of a fusion bomb comes from. Trying to get the fission to happen quickly and completely impacts the yield of the second stage. This housing blows itself apart from all the heat, and this is where most of the fallout comes from the bomb.
How does fission and fusion both release an excess of energy? Because iron is the most stable element. Fuse anything smaller, and you’ll get energy, fuse anything bigger, and it’ll cost you energy. Split larger elements, and you’ll get energy, split smaller, and you’ll lose energy. It’s why stars go supernova a fraction of a second after they start fusing iron, it’s an energy sink that causes the collapse, the explosion of a star is just the rebound.
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