Nuclei of certain heavy elements, such as plutonium or uranium, can sometimes split into smaller parts spontaneously. This is called nuclearfission, which involves the release of neutrons and energy.
Fission can be induced by neutron bombardment. The fission of a uranium isotope with an atomic mass of 235 or a plutonium isotope with an atomic mass of 239 brings about the release of 2 or 3 neutrons with each fission event. The neutrons cause the fissure of further nuclei and therefore start a chain reaction. The energy thus released may be used for peaceful or military purposes.
Uncontrolled chain reaction
In nuclear bombs the chain reaction is uncontrolled. It is a self-propagating process, since the 2 or 3 neutrons released during the first fission event cause the fissure of many nuclei. The chain reaction is induced by spontaneous fission when the mass of the fissile material exceeds the criticalmass. The energy released causes an enormous detonation, so a single bomb can destroy an entire city.
Controlled chain reaction
In nuclear reactors the chain reaction is controlled, as the control rods absorb a certain amount of neutrons. The reaction can be accelerated by pulling the control rods out, and slowed down by pushing them further in. A few grams of fissile material provides enough energy to satisfy the yearly energy demand of an average family.