An understanding of these principles and their manipulation is the basis for the design of both weapons and power plants.

Nuclear power reactors are designed to use the controlled fission of enriched uranium to heat water which will produce steam. The steam is used to turn turbines just as it would be in a coal- or gas-fired power plant. The turbines are attached to generators which produce electricity.

The fuel core of a power plant contains more than a critical mass of 3% enriched U-235. Moderator or control rods containing cadmium (which absorb neutrons) are inserted among the fuel rods until the neutron capture ratio is 1. In U.S. commercial power plants ordinary water is also used as a moderator and heat exchanger. Slowing neutrons down means that a smaller critical mass is needed.