A weapon deriving its great explosive force from the sudden release of Nuclear energy through the fission, or splitting of heavy atomic nuclei.

Practical fissionable nuclei for atomic bombs are the isotopes of Uranium -235 and Plutonium -239 which are capable of undergoing chain reaction. If the mass of the fissionable material exceeds the critical mass, the chain reaction multiplies rapidly into an uncontrollable release of energy.

An atomic bomb is detonated, by bringing together, very rapidly (e.g. by means of a chemical explosion) two sub critical masses of fissionable material. The ensuing explosion produces great amounts of heat, a shock wave and intense neutron and gamma radiation. The region of the explosion becomes radioactively contaminated, and wind-borne radioactive products may be deposited elsewhere as fallout.