Especially in terms of the Tsar Bomber – a relatively large weapon – which caused the biggest nuclear explosion ever. If nukes deal with atom sized particles, why does a bigger sized weapon = bigger explosion?
Unsure if correct flair but when dealing with nuclear weapons a few could apply
In: Engineering
The yield is indirectly related to the actual size of the weapon, because we’re dealing with nuclear chain reactions here, the amount of fissile material determines the yield. But, the fissile material has to be close to more fissile material to continue the chain reaction. What will happen is that the core of the bomb with explode too quickly and some of the fissile material will be pushed out of the core before it has any time to join in the chain reaction. The gadget – the first plutonium based atomic bomb – solved this problem by using conventional explosives arranged in a sphere around the plutonium core to ‘smoosh’ the plutonium together to get a good chain reaction going. Later designs added things like tampers – heavy materials used to keep the bomb from exploding itself for just a few more milliseconds to get an even bigger chain reaction – and neutron guns – parts that inject extra neutrons into the core to speed up the reaction. The development of neutron guns that could be controlled externally led to ‘dial-a-yield’ bombs, where the operators beforehand can actually control how big the explosion will be by setting the neutron initiator to different settings. So more fissile material = bigger bang, that’s self-explanatory, but also you can be tricky with how you arrange the parts to get an even bigger bang out of the same amount of material.
Later designs are thermonuclear – they combine this fission bomb with hydrogen that was suitable to start a fusion reaction. Depending on how the parts are arranged you can have conventional explosives initiate a fission reaction which ignites a fusion reaction in hydrogen fuel and that in turn starts a fission reaction in other parts, for example if you made some of the parts out of uranium. That’s exactly what the Tsar Bomba was, just a very scaled-up thermonuclear bomb. So a bigger bomb can give you a bigger yield, but also, a small bomb can give you a pretty big yield anyway
I am no rocket scientist so what I understand about the size is that it still needs a standard detonation. I suppose the bigger the bomb, the larger the standard detonated explosive material will be needed compared to a smaller nuclear bomb.
For specific data on how the size (notably the mass) of a nuclear weapon correlates to its explosive output, see [this interactive graph](https://nuclearsecrecy.com/betas/yieldtoweight/). It plots all US nuclear weapons ever made, with their explosive power on the horizontal axis, and their mass on the vertical axis. You can see that it is not a perfect relationship, though there is a sort of “maximally optimized” grouping that goes along the bottom of the dots (getting the most “bang” out of the smallest weight). It is noteworthy that there are real exceptions — if you hunt, you can find the first World War II weapons on there, and you can see that they were REALLY inefficient, getting not that much explosive power out of VERY heavy weapons. (If you play the data over time, like a movie, you can see the trends in weapon design.)
In general, you shouldn’t think of the bomb as being _the fuel_. Yes, the fuel can be relatively small, although for large weapons it is still a significant mass and volume of fusion fuel. The complexity and weight of the weapon though is in all of the clever devices necessary to make the fuel react. The atoms do not split or fuse themselves; they need specific conditions to be created, and a nuclear weapon is a device that creates those conditions.
For very-large weapons, like the Tsar Bomba, you are essentially putting bombs-inside-bombs, and the more fuel you add, the larger the apparatus must be to set it off. Fusion weapons in particular compress their fusion fuel using heavy chambers of uranium metal (a “tamper”) — so that adds a LOT to the weight and size of the weapon.
The Tsar Bomba was not an efficient weapon; it was what happens if you take an already large weapon and just scale it up a bit more to make it more explosive. It would be possible to make a smaller weapon of the Tsar Bomba’s size, though again there is an amount of physical fuel that is necessary.
To give you a sense of how the fuel affects the numbers, every kilogram of uranium or plutonium that you fission perfectly releases about 18,000 tons of TNT equivalent. You will probably not fission it all perfectly so that is an upper bound. The equivalent number for a kilogram of fusion fuel is about 50,000 tons of TNT equivalent. So if we imagined a perfect Tsar Bomba, one that released 100 Mt of energy using only fusion reactions (this is not realistic), it would still be two tons of fusion fuel by itself to get that much energy out. In practice, only about 50% of the output would be from fusion, and the other 50% would be from fission. So that’s 1 ton of fusion fuel and 2.8 tons of fissionable fuel (most of it just natural uranium in the tamper). And that’s at the most perfect efficiency, with not a single atom wasted! If the weapon only reacted 50% of its fuel (not a crazy thing), you’d have to double that weight to get the same output. This is without counting any of the apparatus necessary to make it work, just the raw fuel.
So even atoms weigh a lot!
Just to try to add a real ELi5 answer here:
The “explosive” material of the bomb (ie, the plutonium) is only a small part of the size of the overall bomb. The rest of the bomb is a complicated mechanism designed to get the plutonium to explode.
The amount of plutonium does matter, because you still need a lot of “atom sized particles” to make a big explosion, but it’s not what accounts for most of the bomb’s size.
All else being equal in the design of the weapon, more reactive mass means more reaction, which means a bigger boom, up to a point. But the actual design of the weapon plays a much larger part.
Little Boy (Hiroshima) was a gun-type bomb, shot some uranium at other uranium. This is very inefficient, little of the mass reacts. Fat Man (Nagasaki) was implosion, a bunch of explosives focused around a core of uranium, which caused a higher percentage of the uranium to fission.
But other things can change yield. With the Tsar Bomba they left out a fusion tamper, a sheath around the fusion core that not only promotes the fusion reaction, making it more efficient, but itself fissions to add to the yield of the bomb. Had they included the fusion tamper, the yield may have been almost double at about 100 megatons, and the bomb wouldn’t have been much bigger.
Our Castle Bravo nuclear fusion test of the 1950s expected a 6 megaton yield, but we got 15 because they thought a type of lithium they used would be effectively inert, but it instead contributed to the reaction. Same size, bigger yield because of design.
But like everything else in tech, things have just gotten smaller. In a modern bomb, the electronics are smaller, the high-speed detonators are smaller. At the time precisely focusing explosives to achieve reaction was a new concept, and the [soccer ball](https://www.atomicheritage.org/sites/default/files/03_nuclearweapons_02_implosion_03.jpg) was relatively large and crude. Today we’ve gotten much better at that, and can use much less explosives and fissile material to achieve the same result.
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