> to actually get the powerful chain reaction you have to spend a lot of energy bringing pieces of fuel together (to the point that explosives are used for tha

> to actually get the powerful chain reaction you have to spend a lot of energy bringing pieces of fuel together (to the point that explosives are used for that) otherwise they'd just melt and not explode. This isn't really how it works because you are not "putting energy" into the system via explosives.. You are compressing a plutonium core to cause it to become "supercritical". To your point, this is very difficult because if the explosive lenses are not perfectly timed, the core will not compress properly. This could cause anything from a total "fizzle" where the plutonium is spread over a large area due to "conventional explosives" to a yield far below expectations. To get the "powerful chain reaction" you need to hold this compressed "supercritical" core together as long as possible, to consume the plutonium fuel. This is challenging because the fission reactions increase at an exponential rate, so obviously it cant be held supercritical for long. See: https://en.wikipedia.org/wiki/Critical_mass https://en.wikipedia.org/wiki/Critical_mass