The third barrier between the fission products and the environment is the containment building. The containment building is the familiar large dome-like structure which may be seen when approaching or passing many nuclear power plants. At some plants, the containment is located within a building which serves as yet another fission product barrier.

A containment building generally consists of high density, reinforced concrete as much as 5 feet (1.5 m) thick and is built to withstand not only a severe accident, but also a variety of natural and man-made hazards such as earthquakes, tornadoes, and airplane crashes.

The image below depicts the components of a BWR containment building.