Unstable atomic nuclei undergo spontaneous decay involving the emission of gamma rays, electrons (beta particles), helium nuclei (alpha particles), protons and neutron. In this process, only a few elementary particles escape the nuclei of the decaying atom. There is another nuclear process that involves the decomposition of atoms. This process is called nuclear fission.
In nuclear fission, an atom splits into two small fragments of roughly comparable size. As in radioactive decay, fission often produces gamma rays, electron, protons, and neutrons. It also releases a huge amount of energy when compared to decay processes. That energy can be harnessed to create nuclear power or nuclear explosions.
There are natural radioisotopes that undergo fission when struck by a neutron. These radioisotopes are call fissile and some can sustain a process known as a chain reaction when neutrons emitted by one of their atoms causes another atom to fission. The second atom releases a neutron when it fissions and causes a third atom to fission. Thus, like links in a chain, the reaction proceeds from atom to atom. Some natural isotopes such as uranium-235 and plutonium-239 are called nuclear fuels because if they are present in a high enough concentration, they can sustain a chain reaction. They can be harnessed to generate power or nuclear explosions. The amount of energy released in given amount of fissile materials is millions of times greater than the same amount of other common fuels such as coal, oil and gasoline.
Free neutrons sustain a chain reaction. If the number of free neutrons in a fissile material declines, the chain reaction will stop. If the number of free neutrons in a fissile material increases, the reaction will accelerate and run away. The nuclear material either will heat up to the point that it will melt through any container and sink into the soil or there will be a nuclear explosion. If the number of neutrons can be controlled and kept at a constant level, then the sustained reaction can be used as a source of power. Some substances such as water can absorb neutrons and can be used to moderate a chain reaction. These substances are referred to as moderators.
About 2 billion years ago, the percentage of uranium-235 in natural uranium deposits was three percent, much greater than the seven tenths of one percent that it is today. There were uranium deposits in Gabon, Africa at Oklo that were concentrated enough that the uranium-235 could sustain a chain reaction. The ground water moderated the chain reaction and kept it from running away and the deposits generated about one hundred kilowatts for several hundred thousand years. So the first nuclear reactors that produced sustained power were actually created by natural processes in Gabon.