And if so, what factors determine it?
I know that the scale of the explosion is an insane amount larger than the teeny molecules causing it, but I’m wondering if there could ever be nuclear explosions small enough to take out a single house or block, rather than a whole city from high above.
In: Physics
there is. its called the critical mass. you have to have more radioactive material than the critical mass or you wont get a nuclear explosion. Critical mass is 47kg for weapons grade uranium, but only 10kg for plutonium
as for taking out a single house, probiably not, but then, there is no point. its much easier to just use conventional explosives for that
There’s not a *hard* limit, but it gets harder and harder to make smaller ones because of how they work. The bomb works by capturing and reusing neutrons released during the reaction, and a smaller bomb tends to lose more neutrons compared to how many it captures.
There is a lower limit to the size of a nuclear bomb. In order to explode you need a critical mass of fissile material. In fact the Little Boy nuclear bomb that were dropped over Hiroshima was just two halves of a nuclear core which when put together would detonate. The only thing needed is the mass of the fissile material in close proximity.
There are various factors which change how little fissile materials you need though. Things like neutron reflectors lowers the mass needed. And you can compress the material with high explosives to get it close enough together to detonate. So we were able to create significantly smaller nuclear bombs then the Little Boy. For example the US Mark 54 nuclear warhead developed in the late 50s weighed less then 25kg for the entire warhead. That was small enough to be used by infantry and fighter jets. There were also rumors of briefcase sized nuclear bombs, but most of these rumors comes from “dirty bombs” where most of the energy in the explosion would come from the conventional explosives and the nuclear material would primarily be there to irradiate the area to make it uninhabitable.
Check out the shoulder launched ” Davy Crockett”:
https://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device)
There is no lower limit; some bombs fizzle with low yield. There have been accidents that take out a single room. But this is not an economical use of materials.
There are two
The complicated one: critical mass. TL;DR is you need a certain mass (the exact amount depending on what radioactive element you use) before the chain reaction starts, otherwise you don’t gain more energy than you lose, so no boom. (Extremely oversimplified, obviously)
The easy one: practicality. Why use a nuke if a grenade does the job just fine and won’t make it so entering the blast zone 500 years after detonation will still give you cancer?
As far as I understand, yes. (I am not an expert, I have my physics BA, but that’s it; I never took nuclear physics or the like).
The main requirement for Sustained Fission reactions is having criticality, which means that any initial Fission reactions will cause additional Fission reactions in a constant rate(this is usually the goal in nuclear reactors). Nuclear Bombs want to take this a step further, and achieve Supercriticality, where additional Fission reactions happen at an increasing rate, thus allowing it to react fast and make a detonation.
This is all dependent on the size and shape of the nuclear material, as this is just a measure of how likely the neutron byproducts are to hit another atom and cause another Fission reactions – the less atoms there are, the less likely to have additional reactions.
If you look up Critical Mass on Wikipedia, you’ll see Plutonium-239 has a Critical Mass of 10kg, so any samples less than that would “fizzle out” without a detonation, and we would probably just get a big dose of radiation. The minimum size of a bomb would likely need to be somewhat bigger than 10kg, which would still have considerable destructive power.
Theoretically if you had something strong enough to compress and contain it you could make a ‘fusion’ weapon consisting of a proton boron atom that generates an energetic alpha particles.
For a thermonuclear weapon of the conventional sort the smallest that could be made would be pretty expensive, as it would require 5kg of Californium 251.
The world’s production of Californium is about 500mg a year. This might take a while.
Then you’d have a hollow sphere of subcritical Cf151. Crush it with a bomb into a prompt critical configuration surrounded by a tamper and fusion fuel and you’ve got a nuclear bomb somewhere between 8 and 12kg that would have a yield equivalent of maybe 5 tons of TNT.
There’s two possible materials for a nuclear bomb’s core to made out of – plutonium and uranium. Although both produce the same result, how they get there is a lot different.
A plutonium bomb works by taking a small, 15-20 pound sphere of plutonium and compressing it with explosives. When the sphere it compressed, the plutonium in it undergoes a phase change. This is similar to a liquid turning into a solid, except in the plutonium’s case, its one type of solid turning into a different, slightly higher density type of solid. The increased density of the plutonium then causes it to explode with 100% reliability.
Because a plutonium bomb is being detonated by an increase in density, rather than an increase in mass, a *pure plutonium* bomb doesn’t really have a minimum size beyond the power of the conventional explosives needed to initiate the nuclear reaction.
Pure plutonium bombs haven’t existed since the early cold war. Most nuclear weapons have multiple cores or multipart, layered cores that are initiated by compressing plutonium, which then detonates everything else. Complex bombs like that do have minimum sizes, since the plutonium starter has to generate enough explosive power to detonate everything else.
A uranium bomb works by taking two chunks of uranium and smashing them together such that they create a sphere that weighs ~115 pounds. That sphere then has a pretty good chance of causing a nuclear explosion and a small chance of just blowing itself apart in a much smaller explosion.
Because a uranium bomb is being detonated by an increase in mass, there is a minimum mass requirement for a *pure uranium* bomb which puts its total explosive power in the same range as a Hiroshima sized bomb.
There are ways you can reduce the minimum explosive power of a pure uranium bomb, such as using something called a reflector, which is then explosively forced onto the surface of the core. Stuff like that presents significant engineering difficulties and there will always be an unavoidable chance that the bomb will fail to properly detonate.
This is due to the fact that a bomb begins detonating in a random part of the core. With a uranium bomb, that detonation can begin while the bomb is still in the process of “assembling” itself (ie, while the two uranium parts of the bomb are traveling towards one another in a >115 pound bomb or while the reflector is still traveling towards the core in a <115 pound bomb). If that happens, the bomb won’t properly assemble or explode. This is a significant part of the reason why uranium cores haven’t been used since Hiroshima.
Science wise, yea but tactical application wise, no because the US (of course the US) made the Davy Crockett, which was basically a nuke launcher slightly larger than a bazooka to be used at company level.
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