When describing the Magnox nuclear reactor we explained, on page 81, what is meant by a chain reaction and how it occurs when a mixture of uranium-235 and graphite moderator reaches a critical size. We also explained how the speed of this reaction is safely controlled by the use of boron rods which may be raised or lowered into the core.
In the case of pure uranium-235 alone, a chain reaction is possible without the need of a moderator, provided the uranium exceeds a critical size. The first atomic bomb, used in August 1945, contained two pieces of uranium-235, each of which was not large enough to sustain a chain reaction on its own. When these were forced into contact with one another, they formed a single lump which exceeded the critical mass.
As soon as this occurred a chain reaction started and proceeded at such a rate that a large proportion of the nuclei disintegrated almost instantaneously with tremendous destructive violence. Moreover, this was accompanied by the release of radiation and radioactive by-products which wrought incalculable biological damage.
Since then, bombs and missiles have been made containing plutonium, another fissile material which is a by-product from nuclear reactors. Furthermore, a hydrogen bomb based on nuclear fusion was tested in 1952 and shown to be of much greater destructive power than the uranium bomb. Nuclear fusion is mentioned under thermonuclear energy on page 82.