Uranium is an extremely heavy metal, heavier than gold, and it has the largest atoms of any natural element. Moreover, the atoms have far more neutrons than protons, which does not make them easier to split, but does have an important bearing on the amount of energy they release during an explosion.
There are two 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, which has 92 protons and 146 neutrons (92+146=238). But mixed in with this is about 0.6 per cent of the other isotope, U-235, which has the same number of protons but only 143 neutrons. 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 in the course of time. The spare neutrons are thrown off, and so are various other particles. Left to themselves, uranium atoms eventually lose so many particles that they turn into a completely different element, the metal lead. This change takes many thousand years before a measurable number of atoms have turned to lead.
Atoms of U-235 can be made to break up much faster than this in a chain reaction. Instead of disintegrating slowly by themselves, the atoms are forcibly split by neutrons forcing their way into the nucleus. A U-235 atom is so unstable that a blow from a single neutron is enough to split it. Usually it splits into two smaller atoms of different elements, such as barium and krypton.
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 one which is most harmful to life. It also gives off two or three of its spare neutrons, which are not needed to make the barium and krypton atoms. These fly out with sufficient force to split other atoms if they hit them.
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. This is why the reaction is called a chain reaction. It happens with great speed; all the atoms split within a millionth of a second.
In practice, it is not quite so simple to start a nuclear explosion. 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 insure 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 lb (1 kg) of pure U-235. In practice, however, the degree of purity is so low that 110 lb (50 kg) – the effective critical mass – is required in order to sustain a chain reaction.