ELi5: Which is the difference between an atomic bomb and a nuclear bomb?

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ELi5: Which is the difference between an atomic bomb and a nuclear bomb?

In: Chemistry
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There is technically no difference, they are one and the same. “Atomic bomb” does tend to conjure up images of older, simpler weapons, but that’s just a matter of perspective. “Nuclear weapon” is a little more technical (like if you look at official descriptions of these things) and common for modern day weapons.

Nothing. They are both terms for an explosive device that derives its force from nuclear fission.

A thermonuclear bombs/hydrogen bombs use fusion.

No difference. Back in the day, “Atomic” was the buzzword. Nowadays, the word “nuclear” is used.

They are synonymous. The two terms are both used to describe the same types of devices. There is a third term, thermonuclear bomb, which is a type of atomic bomb or a nuclear bomb that is different. Most nuclear bombs are based on fission between heavy isotopes, typically plutonium and uranium. However thermonuclear bombs get their power from the fusion of light isotopes, hydrogen, helium and lithium. Thermonuclear bombs are much more powerful for their size. But all are still referred to as atomic bombs or nuclear bombs.

Generally “nuclear bomb/weapon” is the broad term that encompasses all weapons that derive their energy from nuclear bonds rather than chemical bonds

Atomic bombs generally refer to fission only devices like Fat Man and Little Boy that are solely powered by the fission of uranium or plutonium. These are big devices and are relatively weak (< 100 kT)

Thermonuclear/Hydrogen bombs get a boost from fusion of hydrogen(generally tritium which is 1 proton + 2 neutrons) and are significantly more destructive. Big Hydrogen Bombs like the Tsar Bomba get up to 50 MT but are massive devices(27 tons), the other end is stuff like the US W76 warhead which is only 100 kT but also only 95 kg so you can put it on a relatively small sub launched missile rather than having to use a massive bomber to drop it.

There are differences in different types of nuclear weapons. The wording itself that you are referring to doesn’t differentiate one type from another.

An atomic bomb is a fissile material that is compressed in a short duration. The first such weapon was a gun-type uranium bomb. An enriched U-235 projectile was fired down the barrel of a cannon into an enriched U-235 plug at the end of that cannon. This was “Little Boy”, a cannon bomb – basically. Sounds ridiculous, sounds absurd, but it worked. And it’s amazing that it worked, because it wasn’t even tested before it was dropped, but the scientists were so sure it would work.

But why did it work? What does compression have to do with it? A fissile material radioactively decays and releases more neutrons than it absorbs, typically 2:1. This means that you have some free, energetic neutrons that can bombard nearby nuclei and cause them to split. The rate of decay can double every cycle – so one atom splits two, which causes four to split, which splits eight, which splits sixteen… All in a fraction of a second. What’s important, then, is you keep the density of your fissile material low before you detonate it, so random decay doesn’t reduce the shelf life or melt it, and then when you do detonate it, you make the density really high so that those free neutrons are basically guaranteed to hit a nucleus.

So fire a projectile into a plug, the density goes sky-high, and in a fraction of a second, a few hundred random decays exponentially grow into a very large, exothermic, nuclear chain reaction. You get an explosion.

They tried to make a gun-type with plutonium, and that was project Big Boy, but it never left the design stage – I think they did make the cannon barrels, but the bombs would be so long as to be impractical. The advantage, and the reason they pursued the gun-type is it’s so simple it’s stupid, it’s cheap to develop and reliable.

Enter the implosion style, which is what all modern nuclear weapons are. You start with a hollow pit, keeping the material away from itself to prevent meltdown and extend shelf-life. They even fill the pit with a chain of beryllium to absorb neutrons, and pull it out when arming the bomb. You surround that pit with shaped explosives, and what happens is you ultimately produce a shockwave that will crush the pit down into a uniform, dense sphere. Density goes up, random decay, exponential growth, etc… The pit doesn’t have to start out as a perfect sphere, in fact, most are some sort of ovoid shape, so long as they crush down to a sphere.

We’re still talking atomic bombs here. This is all WWII stuff. Fat Man was a Pu-239 and Pu-241 implosion type, the same type tested at Trinity.

Now, you can _Boost_ these weapons, and indeed many new nuclear powers like India, Pakistan, and North Korea all have been testing boosted weapons. This is simple, add more free neutrons and energy to accelerate the start of exponential growth. They do this by injecting super-heated hydrogen into the pit a fraction of a second before the implosion, since some nuclei of hydrogen are just neutrons. You only need a few hundred to boost a weapon effectively.

They also use flash tubes, a primitive X-ray tube device to inject more energy into the pit, and they use neutron and gamma ray reflectors that surround the pit to keep as much energy in the collapsing core for as long as possible. They can also select a tamper when assembling the weapon, a piece of metal between the conventional explosives and the pit; this is air gapped from the pit, and makes for a more uniform compression wave when it accelerates across the air gap and crushes down on the pit. One made of uranium will add more fissile material. And they use styrofoam, which turns into an electromagnetically charged plasma, reflecting energy back into the pit.

These things allow for a dial-a-yield in the weapon. Changing the configuration or the timing of some of these boosters can reliably change how much yield a weapon generates.

Now we get to thermo-nuclear weapons. Fusion weapons. You use an atomic bomb, maybe boosted, as above to be the compressor of a second stage. The second stage is a paint can made of U-238, which isn’t fissile, but doesn’t have to be. The bucket is filled with lithium. When irradiated and compressed, the lithium fuses, releasing neutrons. Fast neutrons. Energetic neutrons. What they call thermal neutrons. They’re moving at 17% the speed of light, and flash right through the compressed uranium housing, instantly causing them to split. This is how you get the big booms into the megaton range.

What’s curious is iron. When you fuse small elements, it takes fewer sub-atomic particles to make a bigger atom than you had with the two smaller atoms. When you split larger atoms, it takes fewer sub-atomic particles to make the two smaller atoms than it did to make the bigger atom. Iron is the point on the periodic table where both these statements meet. Iron is the most stable element. Split iron and smaller atoms, and its net energy negative, so too with fusing iron and larger atoms.