One of the articles I read in the World Book encyclopedias last weekend was about nuclear weapons. I learned that there are two main types of nuclear bombs—gun-type and implosion-type—and how they work. First off, the minimum amount of fissionable material needed for these to work is called the “critical mass”.
Gun-type: a piece of subcritical material is placed at either end of the “gun barrel”-like object. Behind one of these pieces is a conventional explosive. When the explosive is set off, the two subcritical masses are pushed together to form a supercritical mass. This type of bomb was used in Hiroshima in WWII.
(The supercritical mass goes through nuclear fission, which is the release of energy when a neutron hits the nucleus of a uranium or plutonium atom. And there’s a chain reaction if there’s enough fissionable material around (critical/supercritical). If there isn’t enough fissionable material around (subcritical) then the chain reaction won’t continue on long enough.
Implosion-type: a ball of subcritical mass is surrounded by some conventional explosives. The explosives are set, off, compressing the subcritical mass into a high-density supercritical mass. This type of bomb was used in Nagasaki in WWII.