We speak of a nuclear reaction whenever the nucleus of an atom undergoes any change in its properties. For example, this could be the loss of one or more protons or other particles from within the nucleus, which in turn is possibly caused by the impact of other particles. In nature this process can take place spontaneously in certain substances and gives rise to radioactivity.

Radioactivity was discovered in 1896 by the French scientist Henri Becquerel who proved that pitchblende, a mineral that contains uranium, could darken photographic plates even if they were wrapped in dark paper. It became evident to Becquerel that a very penetrating form of radiation was involved.

We now know that this radiation consists of alpha particles and that radioactive materials also give out two types of radiation: beta and gamma. Alpha particles are not very powerful and they can be stopped by a thickness of a few sheets of paper or by a few centimeters of air. Beta rays are more penetrating but can be stopped by thick cardboard, a few meters of air or thin sheet metal. Gamma rays, like X-rays are extremely penetrating and can be very dangerous to plant and animal life. To stop them several centimeters of metal thickness is needed to reduce gamma radiation to an acceptable level.

It was not simple to produce these rays artificially and it took many years of difficult research and complicated experiments. In the end scientists succeeded. They bombarded the atoms of certain materials with particles taken from naturally radioactive material. By increasing or decreasing this bombardment, the scientists were able to break apart the protective shelf of electrons and reach the nucleus of an atom.

In this way nuclear fission, or the splitting of the atom, was achieved. Under such bombardment the atom nucleus splits into two smaller nuclei. As this happens, some neutrons are ejected by the splitting atomic nucleus and collide with the nuclei of neighboring atoms. This sets off a chain reaction, releasing enormous quantities of energy which can go out of control with disastrous results.