Matter around us is made of atoms, either alone or, most often, assembled into molecules or crystals. Each atom has a nucleus, where almost all the matter is concentrated, surrounded by a "cloud" of electrons, each of which carries a negative electric charge.

The nucleus is a collection of particles having the same mass, positively charged protons and neutrons which carry no electric charge. There are as many protons as electrons: the atom is electrically neutral. The number of protons within a nucleus, its total electric charge, determines the chemical nature of the atom, from 1 (hydrogen) to 92 (uranium). Two atoms having the same number of protons – hence, the same chemical nature – but not the same number of neutrons are called isotopes of the same element. The atomic mass of a given isotope is equal to the total number of its protons and its neutrons.

   
   
           

During chemical reactions, atoms exchange electrons in order to form, delete or transform molecules. During nuclear reactions, the constitution of the nuclei themselves is altered. The forces binding together the constituents of a nucleus are a million times stronger than the forces linking electrons to the nucleus.

The nucleus of some isotopes is unstable: it must get rid of an excess energy by emitting one or several particles (electron, photon, helium nucleus…): it experiences radioactive decay. Such isotopes are radioactive. Each radioactive isotope possesses a specific half-life, the time needed for half of any given number of isotopes to have decayed.

When the nucleus of some uranium or plutonium isotopes absorbs a neutron, this nucleus becomes violently unstable, splits into two parts of unequal masses and emits two or three very fast neutrons. This phenomenon, called fission, released a huge amount of heat: fissioning one gram of uranium produces more energy than burning one metric ton of oil. Those isotopes that can undergo fission are called fissile, and the two fragments, radioactive in most cases, are called fission products.

Neutrons emitted during a fission reaction may in turn, under proper circumstances, be absorbed by other fissile nuclei, thus causing a fission chain reaction. It is the fission of nuclei present within its fuel which produces the energy generated in a nuclear reactor or plant.