The Atomic Bomb

An atomic bomb consists of two subcritical^* uranium or plutonium hemispheres. The hemispheres are surrounded by chemical explosives and the so-called tamper, which is a very heavy material that is used to delay detonation until a great number of uranium or plutonium atoms are subjected to the chain reaction.

When the explosives are ignited, the two subcritical masses are pressed together. The tamper prevents premature evaporation and reflects pushing out neutrons -> number of nuclear fissions increases -> uncontrolled nuclear chain reaction -> bomb explodes -> within a few minutes: mushroom cloud.

^* The mass at which uranium or plutonium explodes is called the critical mass.

Uranium: 15 - 20 kg

Plutonium: 5 - 10 kg