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’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.
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