There are basically two types of process leading to the large-scale liberation of the energy locked in the nucleus of the atom: fission and fusion or, more simply, breaking up large nuclei and sticking together small ones. In the fission process, a nucleus of uranium or plutonium breaks up into two approximately equal parts, with emission of energy. A few neutrons are also emitted, and these can cause more fissions in the surrounding material, thus maintaining the reaction. Energy is also released if two nuclei of very light elements like hydrogen or helium coalesce; this is called fusion. This process will only take place if the combining nuclei have sufficient energy to overcome the powerful electrical repulsion tending to keep them apart, and this means that the hydrogen has to be at a very high temperature before the reaction will begin. The required temperature is many hundred of times greater than the highest attainable by purely chemical means but it can be produced by a nuclear fission reaction. A fission bomb is therefore used to detonate a surrounding mass of hydrogen. The resulting thermonuclear reaction, as it is called, is self-propagating until the reacting mass is blown apart by the force of the explosion.

It is usual to express the power of a nuclear bomb in terms of the weight of T.N.T. that releases the same amount of energy. The fission bombs dropped on Hiroshima and Nagasaki were equivalent to about 20,000 tons of T.N.T. and since then fission bombs of up to about twenty-five times this power have been developed, together with a wide range of smaller weapons for tactical use.