The principle of nuclear fission. Nuclear fission is a process in which the nucleus of an atom, typically a heavy element like uranium or plutonium, is split into two smaller nuclei, releasing a large amount of energy in the process.

In a nuclear reactor, controlled nuclear fission reactions are maintained in a chain reaction. The core of the reactor contains fuel rods that contain the fissile material, such as enriched uranium or plutonium. When a neutron strikes the nucleus of a fissile atom, it causes the atom to become unstable and split into two smaller atoms, releasing additional neutrons and a large amount of energy.

These released neutrons can then go on to strike other fissile atoms, causing them to split and release more neutrons and energy. This chain reaction is carefully controlled by controlling the number of neutrons available for the fission reactions and by using control rods made of materials like boron or cadmium, which absorb neutrons and reduce the rate of the reaction.

The heat generated from the fission reactions is used to produce steam, which drives a turbine connected to a generator. This generator then produces electricity. The steam is then condensed back into water and recycled back into the reactor to be heated again.

In addition to the fuel rods and control rods, nuclear reactors also contain other safety systems and structures to prevent accidents and to contain radiation. These include coolant systems to remove excess heat, containment buildings to prevent the release of radioactive materials, and emergency shutdown systems to quickly stop the chain reaction if necessary.

Overall, nuclear reactors are highly complex systems that utilize the energy released from nuclear fission to generate electricity in a controlled and safe manner.