Nuclear power involves the use of nuclear fission reactions to produce energy, typically through steam generation. The fuel of a reactor is typically uranium-235, where induced fission neutrons initiate the reaction. One possible reaction is: The produced neutrons collide with other nuclei of uranium-235, i.e., the reaction is self-sustaining, or a chain reaction. The minimum mass of U-235 to cause a chain reaction is called the critical mass.
The neutrons produced are too fast and must be slowed before initiating more reactions, since U-235 will only capture neutrons that don’t exceed a certain speed. These are slowed using a moderator, the material surrounding the fuel rods.
Heat exchangers are used to extract heat from the moderator. Control rods absorb excess neutrons, thus reducing the rate of the reaction.