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
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!
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