Nuclear fission is the energy production method for all currently operating nuclear power plants. When an atom undergoes fission, a neutron splits the atom into two smaller fragments, at the same time emitting two or three other neurons. These neutrons are then free to go and induce additional fissions, which will produce still more neutrons. If the two or three neutrons produced in fission go on to induce only one additional fission on average, then the material is said to be critical. A supercritical assembly on the other hand, occurs when each fission on average triggers more than one later fission. A supercritical assembly is unstable and results in a rapid growth of the nuclear reaction rate. The supercritical assembly must be controlled by either active or passive controls on the reaction to bring the material to the desired critical state where the power output is constant and efficient.

Controlling the temperature in a power plant is extremely important. Left alone, the nuclear chain reaction would create so many neutrons and release so much energy that the temperature would soar. The plant’s circulating coolant alone is insufficient to cool the reactor so nuclear plants control the temperature by regulating the flow of neutrons created.

To do this, control rods composed of neutron-absorbing materials are inserted into or around the fuel rods of conventional nuclear cores. The control rods absorb neutrons thereby controlling the fission and thus preventing the excess release of energy. These control rods can be inserted, removed and positioned as necessary to maximize the efficiency of the power plant .

The energy released from nuclear reactors is usually used to heat water into steam, with the steam then used to turn a turbine and produce electricity.

Source: http://www.dauvergne.com/technology/nuclear-overview/