Background to Nuclear Weapons

The energy released by a nuclear weapon originates in the nucleus of the atom. In the fission bomb, the process involved is the splitting of Uranium or Plutonium nuclei into lighter fragments, fission products. In a thermonuclear or hydrogen bomb, nuclei of heavy hydrogen isotopes - deuterium and tritium - are fused together at the very high temperatures triggered though the fission process. 

The speed of the nuclear reaction is enormous. Both in a fission and fusion explosive, the entire nuclear energy is released in about a millionth of a second. It is thus possible to release by one nuclear weapon more energy in one microsecond than that from all the conventional weapons in the world. 

In order to sustain the chain reaction in a fission explosion, it is necessary to have more than a certain minimum amount of fissile material, the critical mass. The fissile material has to be brought together very quickly if the weapon is to explode with great force. Conventional explosives are used for this purpose and the fissile material put together, with or without compression, to a size which, for a plutonium bomb, needs to be no larger than a man's fist. The chain reaction is then initiated.

If a fission device is accompanied by the heavy isotopes of hydrogen, the high temperature and pressure triggered by the explosion can cause the fusion of these isotopes into heavier ones, thereby releasing vast amounts of energy. The amount of energy released per kilogram of nuclear explosive material can be four times as large in a fusion device as in a fission device.