What is the largest nuclear bomb that could be built?

Factual Questions
41

I again recommend reading through the Nuclear Weapon Archive site I linked to earlier as it’s a great way of explaining these sorts of things in such a way that even I can understand it. It didn’t come down on tablets from Sinai, but it seems to me to be able to answer a lot of the questions here (at least until Stranger on a Train makes an appearance.)

But yeah, from it, the third stage is fissioned by higher energy neutrons from the fusion reaction. From wiki, D-T fusion yields a neutron with 14.3 MeV. This is comfortably higher than the energy of a neutron release by fissioning either U-238, U-235, or Pu-239. Those neutrons—for reasons I don’t really understand—lack the energy to reliably fission U-238 (or Thorium, as mentioned by smithsb). In other words, they can’t fission the material that you’re making the tamper out of. If U-235 grew on trees, you could use it, but U-238 is roughly 300 times as prevalent. So you use it, instead.

Fusion neutrons can fission U-238 though. Accordingly, the fusion reaction, while providing a good deal of energy on its own, is also responsible for fissioning as much U-238 as you care to cover the device with. Now, how exactly do you couple the energy from the primary fission stage (or boosted fission) to the fusion secondary so it can start fusing? That’s still a secret, as far as I know.

My question in my first post was whether you could take the energy from this fissioning third stage, and use it to fuse a much larger supply of fusible material. As it is, we can only make the primary so large—call it about a megaton, though the link hints that more is possible—and therefore we can only make the secondary stage so large. Call it roughly 50-100 Mt. We can use that energy to fission a tertiary stage, but if we want to start moving asteroids or other things that you’d want a 1 Gt yield, wouldn’t it be nice if you could get another fusion stage?

Ironically, due to militaries getting away from very large weapon yields, there’s a thought that there’s no need for a fusion stage anymore. IOW, you can get to the 100-475 kT yields through a fission or boosted-fission weapon, so why have a thermonuclear stage at all? The link goes through some safety concerns with having a large, pure fission weapon. For one, large amounts of fissionable nuclear material have the potential of forming an unintentional critical mass. Again, as smithsb noted, this is undesirable.

42

I’ve always though they must have blown a lot of ozone away with the 1000s of nukes tested and deployed.

Or is it too high up?

43

This brings up something else.

When your chunk of Uranium or Plutonium gets too big its going to have to be either less enriched Uranium and/or not all Plutonium to keep from being critical or near critical just from sitting around. So, an increasing percentage of the material isn’t going to be the good stuff so to speak. This other material may make it harder to get a working implosion going or decrease the offerall efficiency of the bomb as well.

So, it could also be like the dimishing returns scenario. Except your bomb does keep getting bigger and bigger but you are getting less and less efficiency out of it.

44

I don’t understand what you’re trying to say. Fusion cannot occur at earthly temperature and pressure, so no one can make fusion happen here without super high temp and pressure because that’s the way the universe works.

Fission is the easiest way to get that temp and pressure.

Saying “we don’t have fusion, we have fission/fusion” is a triviality like saying “I don’t have a car, I have a battery/car system” or “we don’t have true high explosives, all we have are blasting cap/high explosives.”

45

But fission is not the only way to get that temp and pressure - plenty of researchers produce fusion without fission, using lasers, plasma pinch or ion acceleration. These processes are not yet over unity, and cannot produce a catastrophic release of energy (as in a bomb). But it is theoretically achievable, and would be pretty desirable (very clean energy release) - we just haven’t got there yet.

Si

46

Damn! I forgot to pay mine this month!
Thanks for the reminder. :slight_smile:

47

You’re still describing opportunistic fission of the secondary (or tertiary) tamper.

Go read 4.5.1.4 High Yield and Multiple Staged Designs

It specifically defines multistage weapons and times you’d want to use them.

48

As far as this thread/OP is concerned, as I take it, this is pertinent and interesting, but is not of the essence.

That being said (:)), IIRC, Rhodes mentions a last-minute (!) concern expressed by Gen. Groves, I think, on whether the atmosphere of the entire planet would catch fire. Fermi did some math and reassured him it wouldn’t. Yet two physicists, famous ones even to me, made a bet on whether it would or not.

Does any of that sound correct?

Well then, at what point would our super weapon (not what Teller initially called the H-bomb, “The Super”) take out the atmosphere? By feeding the subsequent oxygen combustion, or by blasting it the hell away? Or by some other process?

Now that’s a bomb.

49

Rhodes covers this in “The Making of the Atomic Bomb”, page 664. It was Enrico Fermi who offered to take bets on whether the Trinity Test explosion would ignite the atmosphere. This annoyed Groves, apparently. Another scientist, Bainbridge, was angry at Fermi because he thought the joke might frighten nearby personnel who did not have the scientific background to know that it was extremely unlikely.

The Trinity device was a plutonium fission weapon, the type that was later used on Nagasaki. As for whether a fusion weapon could ignite the atmosphere, I take it that Teller’s assertion about the 100 megaton practical limit means the answer is no.

50

Thanks. Well, I certainly screwed that recollection.

51

I don’t think size is the key factor in igniting the atmosphere. The calculations they did showed that the actions inherent in a nuclear reaction were highly unlikely to cause the atmosphere to ‘catch fire’ and a bigger bomb wouldn’t change that. By the early 60s scientists and military planners knew that bombs much bigger than 10MT weren’t militarily any more useful than smaller ones. In fact, before ICBMs were perfected, they were *less *useful because it made it more and more unlikely that the bomber crews would survive delivery of them, which could call into question their commitment and discipline if they all knew they were on suicide missions!

The Russians knew this too, so most of the late multi-megaton tests were just US & USSR saber rattling. The Tsar Bomba most certainly was. Even its prefix ‘Tsar’ here refers to something which is so grandiose & over-sized as to be useless.

52

Although there are practical limits (weight and size for instance), there is no theoretical limit to the yield of a thermonuclear weapon. A USAF classified study, quickly discarded as being impractical (thank God) and since declassified, theorized that the Saturn V Rocket (the same one used for the NASA’s Apollo Missions to the Moon) could carry a 700 Megaton H-Bomb into a F.O.B.S. (Fractional Orbital Bombardment System) trajectory capable of attacking any point on Earth. I hope not too much taxpayer money was spent on this.

53

At least we know how to deal with the zombie apocalypse …

(this is an old thread, round these parts called a zombie - you will get zombie jokes)

54

The National Ignition Facility in Livermore, California uses the pressure of laser beams on a small pellet to trigger a small fusion reaction, no fission required. There are other kinds of fusion reactors that do not use fission.

55

If you could bury the device at the bottom of a deep shaft, say two miles underground, it could be very powerful. The surrounding rock would provide enough tamper to contain the explosion long enough for a lot of reaction to occur. If you wanted, say, to make a crater three miles across and blow Tambora amounts of dust into the atmosphere, this would do it.

56

Sure, but thats a fusion reactor not a bomb. AFAIK we can’t make fusion bombs that don’t use fission to start the fusion.

57

How big do they have to get before they crack the earth like an egg? Seriously.

58

Cite? I’ve seen references to proposed military missions for the Saturn V, but not as a nuke carrier. The only reference I could find was a Wiki article saying that the payload of a Saturn V translated theoretically into a 700 Megaton device.

59

From post #21 of this thread:

If that’s correct, I don’t think we have to worry about splitting the planet in half.

60

The Earth isn’t held together because it’s a solid lump of rock. It’s held together by gravity. If you could take a giant laser beam and slice the Earth in half nothing would happen, the two haves would just smush back together again.

The way to think of it is, to crack the Earth apart you have to throw material into space at faster than Earth’s escape velocity. To do this to a large fraction of Earth’s material would require an awful lot of energy. No, not the amount you’re thinking of, even more than that. Lots more. Keep going. You’re still not even close.