We seem to have hit the limit with tests such as Tsar Bomba, but that is only because it’s not reasonable or necessary to push it any farther. But I’m wondering what the theoretical limit is. Is there a limit for a single device in which size would make an effective reaction impossible?
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We can make them as big as we want. The teller-ulam design allows us to chain larger and larger fusion bombs as much as we want.
The only real “limit” is that big nukes are less effective than a few smaller nukes.
If a nuclear bomb gets too large, it will collapse into a black hole. Therefore, there is definitely a limit to how large a nuclear bomb can get.
No. Fusion, under the right conditions, is a runaway process, and it’s limited only by its own ability to maintain the heat and temperature necessary for it to continue.
In fact, [for a few minutes at the very beginning of the Universe](https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis), the *entire Universe* was undergoing fusion at massive rates. During that time, about 25% of the Universe’s regular matter (that is, the part made of protons and neutrons) fused from hydrogen and free neutrons into helium. By a very wide margin, the majority of the fusion that has ever happened in the Universe – despite all its trillions upon trillions of stars fusing for eons – occurred in that 20 minute window in the infant Universe.
Obviously engineering such conditions is far beyond the power of humanity, but it is not (in principle) impossible.
Depends what you mean by a “device”.
The sun has been undergoing continual nuclear burning that makes the Tsar Bomba look like a pea shooter, for 4.5 billion years. And our sun is not terribly big – there are *natural* explosions (supernovae) that release more energy than an entire galaxy’s output, for a brief period, and would wipe out all life on any planet within a few dozen light-years or more.
There’s no *theoretical* reason why we couldn’t make a device that would push enough Hydrogen and Helium together to create a supernova anywhere we wanted one, but obviously that’s an engineering problem that’s way *way* beyond what we can currently do.
A fission bomb has radioactive material which means it’s constantly losing some atoms to decay, but it’s typically slow enough that you don’t have to worry too much. I wouldn’t keep a plutonium core around the house as an accent piece, but brief exposure is no real issue. This decay is entirely spontaneous (no trigger)
The thing that’s serious is some of the expelled particles have enough energy if they hit another nucleus, they can kick-start the fission of that atom too. Normally, this is so rare that it doesn’t make a difference. Criticality is the term used to describe when enough mass of the core is in a small enough volume that the rate of this triggered decay is increased to a devastating chain reaction. More mass closer together increases the odds of a collision.
So the term critical mass is less applicable than critical density, but they are related. I have to think, though I don’t know if it’s been tested before, that you could make some sort of honeycomb structure or something that allows you to increase the mass without increasing the density. You could make this structure very very large. As long as it’s very close to the tipping point of critical mass, you can use conventional explosives to compress it to criticality, causing the chain reaction and explosion.
Eventually, though, one of two things will happen as you try to make it bigger and bigger. Either the mass of the object will cause its own gravity to overpower it’s structure and collapse under its own weight into critical mass. Or it could be that the structure gets so huge that there’s still enough *stuff* in the way that the ejected particles are still highly likely to collide with another atom eventually. This is less likely since unless there’s a perfect vacuum inside, the air will eventually slow down those particles until they don’t have enough energy to kickstart fission even if it does hit another atom.
There is no theoretical limit. One is limited by fuel, cost, size, and will. There were [designs looked at during the Cold War for 10,000 megaton bombs](https://blog.nuclearsecrecy.com/2012/09/12/in-search-of-a-bigger-boom/). With teraton-range nuclear weapons and an ocean of deuterium, [you could turn a planet in a bomb](https://blog.nuclearsecrecy.com/2018/06/29/cleansing-thermonuclear-fire/).
The issue with all reactions is that you have to engineer conditions that keep them going. But if one imagines hypothetical alien civilizations with resources beyond our present ones, there is nothing stopping one from making absolutely huge weapons, at least to my knowledge.
Tsar Bomba was only tested at half its design yield because it was already ridiculously powerfull. Documentaries claim the shockwave circled the globe three times.
Since its been 40+ years (and meanwhile computers were invented) it’s probably reasonable to say that designs with 10x or 20x the explosive yield are entirely possible with current technology.
So basically we can already make weapons much larger than any practical use calls for.
Maybe someday we’ll want to nuke an asteroid or something and build designs at 100x Tear Bombas yield.
What we can build is only limited by our technology so we’ll probably never know just how big the theoretical maximum is.
Odds are we’ll switch to more powerfull energy sources like antimatter before bothering to build ludicrously large fusion devices.
There is no theoretical limit based on how many stages and boosters are used. But at a certain point it becomes worthless, as the fireball exits earth’s atmosphere and energy is lost into space. Reactions larger than this would have most of the energy completely lost to space, so that’s the practical maximum.
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