Ok, let me take a swing at clearing a couple things up. A subcritical mass means there isn’t enough radioactive material in one “lump” to initiate a nuclear reaction. It is still radioactive, because the isotopes are always decaying and giving off radiation, but there is no chance that it will start a reaction giving off massive amounts of radiation.
A critical mass happens when enough of the material is in one “lump” for a nuclear reaction to begin. Once that happens, you get nuclear fission, meaning the isotopes are not just decaying, steadily giving off “rays” of radiation over time, they are actually splitting apart at the molecular level, releasing huge quantities of energy and radiation every time that happens, and it will continue to happen in a chain reaction until all the fissile material is broken down into less reactive substances.
The raw amount of material is not the only factor, it also has to be in the right state and a proper shape for a chain reaction to start, but since they were building a nuclear bomb, the material they were working with was already in that configuration. All that was needed for a fission reaction to start was for the two masses to come into contact.
That was very understandable even for me lol Thanks. But what is the material? Plutonium? why does a certain amount more cause critical mass? thanks in advance. What an interesting subject