The difference between an nuclear bomb and a hydrogen bomb.
In: Engineering
Fusion and fission, heavy elements like Uranium generate massive amounts of energy when they are split apart (fission) light elements like Hydrogen in our Sun generate huge amounts of energy when they are forced together (fusion). Both use the power of the atom (nuclear power) to generate energy.
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A nuclear bomb is any weapon that uses nuclear fission to generate its energy. A hydrogen bomb is a special version; it uses a nuclear bomb (the “primary”) to compress a special device (the “secondary”) which contains special isotopes of hydrogen which undergo fusion, greatly improving the yield of the overall weapon.
There are atomic (fission) bombs, that release energy through the splitting of heavy atoms. The bombs dropped on Japan were these, and had explosive outputs of around 15,000 tons of TNT. Their fuel is typically either uranium-235 or plutonium-239.
There are also hydrogen (thermonuclear/fusion) bombs, that release energy through both the splitting of heavy atoms _and_ the fusion of light atoms. They work by using the energy released from fission to ignite the fusion reactions, which then usually produce more fission reactions. The first hydrogen bombs, developed in the 1950s, had explosive outputs around 15,000,000 tons of TNT. But you can build them to have outputs more like 150,000 tons of TNT, which is what most modern nuclear weapons are like — small H-bombs, which are very efficient in terms of weight and size, and so you can put many of them on top of a single missile. The fusion fuel can be types of hydrogen (deuterium and tritium), or a type of lithium.
Both of these types of weapons go under the mutual heading of “nuclear weapons.” There are of course many little complexities (you can have fission bombs that use a little bit of fusion but aren’t really hydrogen bombs, because they aren’t using the energy from the fusion so much as just using the fusion to generate some neutrons; this is called “boosting”) not covered by these two categories.
There are two ways to generate nuclear energy: fusion and fission. Fission is one large atom breaking into 2 smaller atoms and some neutrons, and Fusion is 2 small atoms forming one larger atom. Both create a lot of excess energy, although fusion creates much more energy than fission (for the most part). A nuclear bomb uses a nuclear reaction to create energy, then uses that energy to make new nuclear reactions, so on and so forth until the entire bomb’s fuel is consumed and the energy creates an explosion. Because fusion bombs (hydrogen bombs are these) are much more efficient than fission bombs (like the ones dropped on Japan), they create much larger explosions.
A hydrogen bomb is a kind of nuclear bomb. Specifically one that uses nuclear fusion to boost the yield (explosive power) of the bomb.
The first nuclear weapons used purely nuclear fission (splitting an atom) to produce energy. That gives you a huge bang, but it’s pretty inefficient since most of your fuel just gets blasted apart instead of fissioning to make the big boom.
Second-generation bombs introduced a second stage with a different kind of fuel that allowed for fusion (smooshing two atoms into one bigger atom). The fusion stage takes the energy from the fission, and uses it to fuse lightweight atoms into heavier ones, which releases crazy amounts of energy. This makes your bomb much more efficient, which means you get a bigger boom from a smaller bomb.
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.
a hydrogen fusion bomb uses a nuclear fission bomb to kickstart a much more powerful explosion.
Nuclear fusion typically happens under tremendous heat and pressure and such conditions are not naturaly found outside of the center of a star. By wrapping a hydrogen bomb core in a shell made up of fusion bomb material the outer material can be detonated compressing the hydrogen core and mimicing the insane conditions normal only found in stars.
Both are nuclear. One uses fission and makes a small boom. Typically on the scale of kilotons of TNT
One uses fusion and makes a large boom. Typically on the scale of megatons, so about 1000 times more or so
One splits a heavy element into smaller elements and energy is liberated (this is a fission bomb – uranium or plutonium)
One combines a light element into a heavier one and this liberates energy (this is the hydrogen bomb)
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