Fissioning takes place in the reactor core. Surrounding the core of the reactor is a shell called the reactor pressure vessel. To prevent heat or radiation leaks, the reactor core and the vessel are housed in an air-tight containment building made of steel and concrete several feet thick. The reactor core houses approximately 200 fuel assemblies. Spaced between the fuel assemblies are movable control rods.
Control rods absorb neutrons and slow down the nuclear chain reaction.
They are called control rods because they help control the fissioning process. Water also flows up through the fuel assemblies and control rods to remove some of the heat of the nuclear chain reaction. The nuclear reaction generates heat energy just as burning coal or oil generate heat energy. Likewise, the heat is used to boil water into steam which turns a turbine generator to produce electricity. Afterward, the steam is condensed back into water and cooled in a separate structure called a cooling tower.