I cant quite understand critical mass. They say in a fission bomb they have to seperate two blocks of fissile Uranium of subcritical mass so they wont undergo spontaneous fission. What exactly is critical mass? Is it just the amount of Uranium held in one place at one time and if so why does it have to be under the critical mass. Even if it is a small amount and it spontaneously underwent fission would it not cause a smaller chain reaction and blow up that smaller amount of Uranium?? Why is it that a large amount of Uranium in one place is in danger of undergoing spontaneous fission? What would cause one of the atoms to split and cause a chain reaction? Another question I have is when they propel the two blocks of fissile Uranium at each other what is it that causes the chain reaction? I thought the only way to cause the fission of a Uranium was bombarding it with neutrons. Is propelling Uranium particles at each other the same thing as propelling neutrons at the atoms?