Rehashing some of what's been said and adding to it:
A nuclear chain reaction occurs where more neutrons enter into a fissible mass than leave it, where those neutrons trigger additional fission events.
"Criticality" is the point at which that neutron emission is just balanced: the same number are added as are consumed. This is often fairly stable, and can be further controlled with moderating systems (e.g., control rods, circulating water, or neutron reflectors which increase neutron flow). There's also the matter of "prompt" vs. "delayed" neutrons. The first, prompt neutrons, are emitted immediately following a fission event, the latter occur after some delay, from milliseconds to minutes or longer. The ratio of prompt to delayed neutrons also matters in controlling a nuclear reaction.
A nuclear reaction at criticality is not a bomb, at least not necessarily. What it is however is sustained, which is to say that the nuclear reaction will continue unless circumstances change.
A nuclear bomb, and specifically a fission bomb, requires not only a critical mass but a supercritical one, with a large amount of the material going critical at once. The challenge for the engineer is that nuclear reactions release so much energy that the explosive material itself can be blown apart before enough of it has time to react. So the trick is to transition between subcritical and supercritical masses quickly.
For Uranium-235, the reaction is slow enough that a "bullet-style" design is sufficient. A supercritical mass is arranged in two pieces, which are separated until detonation is desired, at which point one (usually smaller) mass is shot into the other, like a bullet down a gun-barrel. Plutonium-239 is so fissile that this would result in premature criticality and only a small fraction of the material would fission before being blown apart. Instead, an implosion design is used, in which a subcritical mass of plutonium is surrounded by explosive charges which, when detonated, compress the core sufficiently that it does achieve criticality, and the much larger nuclear explosion follows.
The Uranium bullet-style device was considered sufficiently reliable that it was not tested. The Hiroshima bombing was the first detonation of this style of weapon. The Trinity test was to confirm the theory of a plutonium implosion-style design, and Nagasaki saw the second explosion of such a weapon.
In the case of the Hiroshima (uranium) bomb, about 1 g of matter was converted to energy, and about 660 g of a total fissile mass of ~51 kg actually reacted, or about 1.3% of the total mass. Essentially the bomb was already coming apart before any more material could engage in fission. See: <https://old.reddit.com/r/askscience/comments/1546rcv/why_did...>
I believe values are about the same for the Nagasaki weapon.
A nuclear chain reaction occurs where more neutrons enter into a fissible mass than leave it, where those neutrons trigger additional fission events.
"Criticality" is the point at which that neutron emission is just balanced: the same number are added as are consumed. This is often fairly stable, and can be further controlled with moderating systems (e.g., control rods, circulating water, or neutron reflectors which increase neutron flow). There's also the matter of "prompt" vs. "delayed" neutrons. The first, prompt neutrons, are emitted immediately following a fission event, the latter occur after some delay, from milliseconds to minutes or longer. The ratio of prompt to delayed neutrons also matters in controlling a nuclear reaction.
A nuclear reaction at criticality is not a bomb, at least not necessarily. What it is however is sustained, which is to say that the nuclear reaction will continue unless circumstances change.
A nuclear bomb, and specifically a fission bomb, requires not only a critical mass but a supercritical one, with a large amount of the material going critical at once. The challenge for the engineer is that nuclear reactions release so much energy that the explosive material itself can be blown apart before enough of it has time to react. So the trick is to transition between subcritical and supercritical masses quickly.
For Uranium-235, the reaction is slow enough that a "bullet-style" design is sufficient. A supercritical mass is arranged in two pieces, which are separated until detonation is desired, at which point one (usually smaller) mass is shot into the other, like a bullet down a gun-barrel. Plutonium-239 is so fissile that this would result in premature criticality and only a small fraction of the material would fission before being blown apart. Instead, an implosion design is used, in which a subcritical mass of plutonium is surrounded by explosive charges which, when detonated, compress the core sufficiently that it does achieve criticality, and the much larger nuclear explosion follows.
The Uranium bullet-style device was considered sufficiently reliable that it was not tested. The Hiroshima bombing was the first detonation of this style of weapon. The Trinity test was to confirm the theory of a plutonium implosion-style design, and Nagasaki saw the second explosion of such a weapon.
In the case of the Hiroshima (uranium) bomb, about 1 g of matter was converted to energy, and about 660 g of a total fissile mass of ~51 kg actually reacted, or about 1.3% of the total mass. Essentially the bomb was already coming apart before any more material could engage in fission. See: <https://old.reddit.com/r/askscience/comments/1546rcv/why_did...>
I believe values are about the same for the Nagasaki weapon.
More on fission weapon designs: <https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html>