In a nuclear power plant, controlled nuclear fission generates heat in the reactor core. The heat is used to produce steam. The steam drives a turbine connected to a generator which produces electricity.

This principle applies to the six pressurised water reactors (PWRs) and two boiling water reactors (BWRs) still in operation in Germany. Both are types of light water reactor, which use low-enriched uranium (LEU) as a fuel and normal water (H2O) as the coolant and neutron moderator, unlike heavy water reactors, which use heavy water (deuterium oxide D2O) as a coolant and neutron moderator.