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
A pure fusion bomb would produce less fallout per unit yield than a fission only bomb but it would still produce some as others have noted fusion bombs contain a primary fission explosion that is required to set off the fusion reaction.
In the real world, most fusion bombs are actually fission-fusion-fission bombs where a primary fission explosion with fissile martials (U-235 or Pu-239) ignites a fusion reaction. The extremely high energy neutrons of the fusion reaction subsequently set off a tertiary U-238 tamper to an enormous fission explosion. Since U-238 is not fissile, you can put as much of it in the bomb as you want without a risk of it reaching critical mass.
The Tsar bomb (the largest manmade explosion ever) had a yield of 50 megatons of mostly fusion. But the original design had a U-238 tamper that would have increased the yield to over 100 megatons but would have produced an enormous amount of fallout. Before the test it was decided to remove the tamper to limit the fallout.
i am not a nuclear physicist so my understanding may be wrong.
a hydrogen bomb is essentially a VERY short lived star stored in a bomb (which is kind of interesting if you ignore the horror of nuclear war) that reaction DOES produce radiation and some fallout but it’s actually a fairly efficient reaction. where most of the fallout in a a hydrogen bomb comes from is the detonator which is a fission bomb that is used to start the fusion reaction which is significantly less efficient.
if you just used the fission detonator as a bomb itself (ie an atomic bomb) you’ll produce a similar amount of fallout but over a smaller area (since the blast wave won’t physically disperse the material as much)
One thing to note is that a thermonuclear bomb needs to use Plutonium or Uranium as the initial stage to produce the heat and pressure needed to start the fusion process of the lithium/hydrogen secondary stage. A significant percentage of the total bomb yield comes from this first ‘atomic bomb’ stage of the weapon. When they detonate there isn’t 100% efficient fission, so a lot of the uranium/plutonium atoms are scattered, as well as a lot of the fission byproducts that also emit significant radiation risks.
So a hydrogen bomb has a fission section which “ignites” the fusion section, but the fusion section is wrapped in a heavy metal. Sometimes this is natural uranium which fissions as well and produces lots of radioactive isotopes plus a lot of the power released by the bomb.
There may be lower fallout designs in use now that don’t have the same wrapper and use radiation instead of blast pressure to kill people without as much infrastructure damage.
There have also been designs that produced extra fallout but these probably haven’t been built.
Just a note.
No bomb is 100% fusion. You need a fission reaction to start the fusion reaction. Most bombs aim for a 50/50 fission/fusion mix. Even then, only about 30-40% of the material is actually consumed by either reaction.
And that material that isn’t used up has to go *somewhere*. Which means some amount of fallout from the bomb itself. Now, a bomb can also turn debris in the area radioactive, and that can get sucked up into the atmosphere and also become fallout. Although with a fairly short half-life compared to the material ejected from the bomb itself. But, either way, it is not good to be around it.
Yes, absolutely. A hydrogen bomb is just a regular fission bomb with extra stuff (the fusion secondary), and the point of the extra stuff is mostly to generate a lot more neutrons to trigger even more fission in the fission core. The fission of U235 is what produces Cs-137 which is the most dangerous fallout product from a nuclear bomb, and a fusion bomb just produces more U235 fission than a fission bomb would on its own.
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 ».
The Castle Bravo hydrogen bomb test was a massive fallout disaster that ended up killing some Japanese fishermen that were downwind from it. The scientists in charge of the test miscalculated the yield due to unknown fusion properties of lithium isotopes. So yes, if the fireball hits the ground, whether through miscalculation or an intentional ground burst, it’s going to suck up whatever is on the ground and mix it with all the nasty radioactive fission products from the bomb.
The Tsar Bomba test was relatively “clean” because the fireball didn’t hit the ground. The nasty fission products are lofted into the upper atmosphere, distributed over the globe and have a lot more time to decay before they fall back to the ground. Partially because the bomb was detonated so high in the first place, Partially because its massive yield lofted the debris cloud so high and Partially because the fission products didn’t have heavier bits of vaporized rock/soil/people/etc to drag them back down before they decayed or disbursed.
Ok so the top comment is saying things both correct and incorrect.
Fusion based bombs do not create dangerous daughter nuclei (on purpose) because they burn deuterium and tritium (via lithium) to produce very energetic helium nuclei.
However, in the case-ulam-teller design (Ivy Mike, and other early H-bombs) there is a uranium or plutonium tamper that encases the fusion fuel. This high-Z material reflects neutrons and radiation back into the fusion fuel.
Basically, to create a fusion reaction with a self sustained burn you need excellent confinement of energy. The easiest way to do this is to make the fuel dense enough that neutrons and photons are reabsorbed instead of escaping the system.
So in the CUT H-bomb a traditional fission bomb is set off to increase radiation pressure and heat the fusion fuel, which then begins to fuse until the kinetic pressure is so great that the encasing radiation reflector explodes.
In short, there is a very large amount of fall out from an H-bombs, but considerably less than a fission bomb of the same explosive magnitude.
*I don’t have any knowledge of thermonuclear bomb design past the 60s because that is all classified
If you line the bomb with led it will reduce the fallout. If you lace it with cobalt you will absolutely end the world. It’s that versatile.
The Russian 50megatonne tzar bomba was lined with lead to reduce the fallout. If they literally replaced lead with cobalt it would have irradiated 1/2 Russia and Europe for 1000 years.
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