Is there any limit, practical or otherwise, of how big a fission bomb can be?
Lets say I have a lump of weapons grade plutonium about the size of my car. Can I turn it into a nuke (I am a mad genius with access to all sorts of modern tech by the way).
Also, if I found this plutonium out in space and then sent it at high speed on a collision course with the moon, would the resulting forces cause the required chain reaction?
A collision alone won’t cause a chain reaction. This sort of thing has to be very carefully engineered. If it doesn’t go exactly right, you get a very small bomb that just sprays the rest of the material harmlessly away without it contributing to the bomb. And besides, this may surprise some people, but the U.S. has accidentally dropped atomic bombs on a couple of occasions. They didn’t explode.
This wikipedia page talks about size limits (scroll down to “Yield limits”):
Fission bombs are very difficult to make very large, since you have to keep the material subcritical until you want to set it off. Fusion bombs don’t have a critical mass, though (at least, not one smaller than the size of a star), so they can be made arbitrarily large. The practical limit to the size of a fusion bomb is that eventually, you’re destroying everything out to the horizon anyway, and there’s nothing else left to destroy.
That isn’t so much of a problem–you just make the bomb in several different subcritical components and implode them together–as is the problem that regardless how fast you attempt to thrust them together with chemical explosives, above a certain level the device will blow itself apart before using any significant amount of the nuclear material, and so adding more material doesn’t help. I don’t know what this level is–it would depend on the configuration of the device–but the largest pure fission device detonated by the United States was the Ivy-King test shot, with a yield of ~500 kT. Using neutron boosting–basically, the fission primary feeds an incomplete fusion process that rapidly generates a mass of neutrons, accelerating the reaction–it is possible to get several hundred kT in extra yield, so I’d guess the largest plausible yield from a boosted fission device is somewhat less than 1 MT.
There is no upper limit to a thermonuclear fusion device–you just keep adding more stages–although I suspect there is a limit on how much additional yield you can tap out of multiple stages versus just deploying multiple devices, and a practical limit on how many stages you can add before the device is unwieldy for practical deployment. Fusion devices themselves have no lower bound, but all known fusion devices utilize a fission or boosted-fission primary, and so that has a minimum size and yield in order to provide enough high energy neutrons to feed a fusion reaction.
Stranger
The Царь-бомба, “Tsar bomb”, was the most powerful nuclear weapon ever built (actually it was the most powerful explosive of any kind ever built). It was designed to have a yield of 100 megatons, but was only tested at half strength (50 megatons) to limit fallout. Only one was ever built. Better missle guidence systems have eliminated the need for bombs of this yield.
Fissionable material is the most expensive single component of nuclear weapons. Either you have to breed Pu-239 in breeder reactors, or sift U-235 out of natural uranium in diffusion plants. Either way we’re talking about materials that have to be accumulated atom by atom. By contrast, fusion devices use (comparatively) cheap lithium and deuterium, and the even cheaper depleted uranium (U-238) left over from the enrichment process for U-235. If you can build a fusion device of modern design, then all-fission bombs are an expensive waste of fissionable material.
As for high-yield fusion devices, there are practical if not physical limits. At about 100 megatons the blast would start blowing chunks of the Earth’s atmosphere away into space instead of propagating along the surface of the Earth. The delivery weight of the device becomes an issue, especially with ICBMs. And since overconcentrating yield in one place becomes redundant, it ends up making more sense to use several smaller devices spread out than one big one.
From a practical weapons effects POV, beyond a certain yield, you’re not doiing any good. “Overkill” is the term of art.
IOW, if I’m trying to destroy an area target like, say, Los Angeles, I’d do a lot better with 10 1MT bombs spread around the megalopolis than one 1 10MT bomb in the geographic center. The single bomb will waste a lot of energy digging a deeper hole at ground zero. And the deeper hole doesn’t do me any good.
Now if I’m trying to destroy a hard point target like, say, Cheyenne Mountain, then I’ll need a bigger single bomb. But even so, I’d do better to send 10 warheads each big enough than to send one warhead ten times larger. In all I get a higher likelihood of success.
So as a result of the above, there hasn’t been a real push to make nukes of ever larger yield. Yields vs. accuracy vs. target hardness are in a more-or-less equilibrium now and have been since the 1960s.
I concur with those above regarding efficiency. I think there are comparatively few nukes in global arsenals of over about 300-500Kt yield. The only real point to anything larger is for cracking deep bunkers.
Back in the heyday of Directed Energy Weapon research for “Star Wars” missile defense programs I heard it proposed, at least semi-seriously, that if one fired a neutron accelerator at a fusion weapon, one might induce a fission chain reaction in the outer (generally U-238) casing, causing the bomb to function “inside out”.
The effect, it followed, would be for the outer fission reaction to contain the fusion and core fission reactions for longer than was the case in a normal detonation, causing the proportion of mass converted to energy to be orders of magnitude greater than usual – sort of a bridge between fusion and antimatter weapons. I recall a projection of a standard 200Kt warhead producing a yield upwards of 14,000Mt
Perhaps the OP would find that more satisfying. 
Whether the specific scenario is realistic or not, the point is that the limitations on bomb yield are due to their physical structure and operations. If one approached the problem from “outside the box”, one might be able to produce greater yields, whatever the intended use might be.
14,000 mega ton ?!
Holy bejeezus
Is that 14 GIGA tons ?
IIRC, in “The Making Of The Hydrogen Bomb” by Richard Rhodes, someone calculated that after a certain point, a fusion bomb is sending a chunk of atmosphere the size on Pennsylvania into space, and anything more powerful than that just sends that same-sized chunk out faster.
While it is true that a high external neutron flux will cause fast fission of the [sup]238[/sup]U tamper, the ultimate effect would not be what you suggest. Rather than amplifying the nominal yield of the bomb, what will happen is that the tamper will fragment and blow itself apart before any significant fusion or containment of neutrons in the Secondary can occur. Similarly, the Primary will also come apart like a cheap gold watch and provide only a fraction of its design yield, resulting in what is called a “fizzle”. This is the principle on which the “neutron enhanced” W66 warhead on the Sprint ABM system worked.
Stranger
I remember hearing about “cobalt bombs” is that some kind of nuclear weapon? Is it just just science fiction?
I’m relieved to hear that the effect isn’t currently feasible or achievable accidentally.
Never actually constructed, but theoretically feasible. In a nutshell – A normal thermonuclear (“H”) bomb is constructed with an outer casing of U-238, which contributes an additional fission reaction to its blast effect. The “Tsar Bomba” mentioned above replaced that casing with one of lead in order to reduce its yield to a “mere” 57Mt. A Cobalt Bomb would have an outer casing made of cobalt, which would produce a particularly virulant sort of fallout.
Jackets of cobalt, zinc, potassium, et cetera could be added to weapons with the intent of producing high levels of long term radiation in order to deny use of the affected zone. Like the “Doomsday Device” in Dr. Strangelove, or How I Learned To Stop Worrying And Love The Bomb, then purpose for this was deterrence and denial. It doesn’t do anything to increase the yield of the device
Stranger
Thanks for in info. I wonder what goes through a man’s mind when making weapons that can kill tens of millions.
I came into mention this. IIRC, it was Teller making the calculation, and he said 100 MT was the limit (perhaps not coincidence that this was the theoretical yield of the Tsar Bomba then) after which your just expending any extra energy moving chunks of the atmosphere around.
“There’s N Millions of them. How can we kill enough of them at once so that they won’t have the will to continue the fight?”
nm
From the movie “Broken Arrow”: