I’ve heard people say there is no fallout, but there typically is a fission bomb in the secondary stage. Where does its radiation go? Is it just blown away by the fusion bomb so it’s no longer as deadly? Isn’t it still there though? Is it just weaker?
In: Physics
TL;DR, there’s an option in an h-bomb’s design to get a massive fission reaction and nearly double the yield for free, so half the yield of most H-bombs is actually a fission reaction, because why not? But this means that nearly half the yield is fission, more than is possible with just an atom bomb, and that means a LOT of fallout.
To elaborate: Hydrogen bombs are composed of two parts: A primary and a secondary. The primary is just an atom bomb, while the secondary is where the fusion occurs.
The exact details of how a hydrogen bomb works are not publicly known, and much of what we know is pieced together from declassified information that has come out over the years. But what we do know is that there is at least 1 fissile component in the secondary which helps drive the fusion (a plutonium « spark plug »). That alone is a small amount compared to the total yield of the fusion, so in theory if you just have that, plus the fact that you are using just one small atom bomb as the primary, means the amount of radioactive isotopes produced is less than the equivalent yield in atom bombs.
But this is the real kicker: The secondary is made of, on the outside, of a heavy metal, which absorbs radiation from the primary and heats up and expands. This creates heat and pressure for the fusion fuel in the middle that drives the fusion reaction. This is called the tamper, and you can make this out of an inert material like lead, but you can also make it out of depleted uranium (left over waste material from enriching uranium to create the A-bomb). If you’re making these bombs, you probably have a lot of DU lying around anyway. And as a bonus, the thermonuclear fusion process is enough to make the DU tamper undergo fission as well.
Since depleted uranium is much more stable than enriched uranium, there is way more depleted uranium in the tamper than would be feasible to put the equivalent in enriched uranium in an atom-bomb before the enriched uranium goes critical. The tamper undergoing fission is can produce a yield nearly as high as the fusion reaction itself.
Now if you’re developing a hydrogen bomb, are you going to leave half the potential yield, for free, on the table? I think not.
But because this last reaction is an absolutely massive fission reaction, it generates an incredible amount of radioactive isotopes. Coupled with the fact that higher yields produce diminishing returns in terms of destructive power as a lot of that energy gets wasted, meaning that more yield is required for 1 hydrogen bomb than the combined yield of multiple atom bombs, and hydrogen bombs can produce just as much fallout as atom bombs for the same amount of destructive power.
That isn’t to say that you can’t create a « cleaner » bomb by sacrificing half the yield, however. This is exactly what was done for the Tsar Bomba test; the original 100 MT yield was deemed way too much and they swapped the DU tamper out with a lead tamper, resulting in a cleaner, ~55 MT bomb. But that was a special case where they needed to reduce the yield; normally, you want the most bang for your buck, and if you want a lower yield then you start off with a smaller bomb. Now compare this with Castle Bravo, a ~13 MT bomb that created the worst radiological disaster ever until Chernobyl. The Castle tests firmly put to bed the idea that H-bombs, at least the designs commonly used, are in any way « clean ».
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