Nuclear weapons are devices that use the energy contained within the nuclei of atoms to create massive explosions. This energy may be released by the fission, or splitting, of atoms or the fusion of atoms, in which small atoms are joined together to make larger ones. Both fission and fusion release huge amounts of energy and radioactivity. Broadly, there are three main types of nuclear weapons. They are the atomic bomb, or A-bomb, which uses nuclear fission, the hydrogen bomb, which uses nuclear fusion, and the neutron bomb, which also relies on fusion.
Nuclear weapons have been deployed only twice in war, at Hiroshima and Nagasaki in Japan. A single atomic bomb was released over each of these cities toward the end of World War II, causing their total destruction.
Nuclear fission
The huge power of an atomic bomb comes from the forces holding each individual atom of a substance together. These forces act over tiny distances deep within the atom itself. Every atom of every substance that exists is held together by them. The energy released by splitting one atom is tiny, but there are so many billion atoms in even the smallest piece of material that a great deal of power can be released from large quantities.
Most naturally occurring elements (pure substances) have very stable atoms that are impossible to split except by using such techniques as bombarding them in a particle accelerator. However, there is one natural element whose atoms can be split comparatively easily: this is the metal uranium. Its special property comes from the comparatively large size of its atoms; they are too big to hold together firmly.
There are two common naturally occurring isotopes of uranium: an isotope is a form of an element distinguished by the number of neutrons in its atom. Natural uranium consists mostly of the isotope U-238, but mixed in with this is about 0.7 percent of the other isotope, U-235. This isotope, unlike U-238, is fissionable (its atoms can be split), and so it is the one used for making bombs.
Both isotopes of uranium, and certain other heavy elements, are naturally radioactive, that is, their big, unstable atoms slowly disintegrate over the course of thousands of years. Atoms of U-235 can be made to break up much faster, however, in a chain reaction. Instead of disintegrating slowly by themselves, the atoms are forcibly split by neutrons forcing their way into the nucleus. When a U-235 atom splits, it gives off energy in the form of heat and gamma radiation, the most powerful form of radioactivity and the type that is most harmful to life. It also gives off two or three free neutrons. These may collide with other nuclei, causing them to fission.
In theory, it is necessary to split only one U-235 atom, for the neutrons from this will split other atoms, which in turn will split more, and so on. In practice, however, there has to be a certain weight of U-235 present before the chain reaction will sustain itself. If there is less than this amount, there will be too few atoms to ensure that neutrons from every atom that splits will hit other atoms. The minimum amount is known as the critical mass. The theoretical critical mass is about 2 lbs. (1 kg) of pure U-235. In practice, the degree of purity is so low that 110 lbs. (50 kg)—the effective critical mass—is required in order to sustain a chain reaction.
Uranium is not the only material used for making nuclear weapons. Another material is the element plutonium, as its isotope Pu-239. Plutonium is not found naturally (except in minute traces) and is always made from uranium.
