Containment building
The diagram shows the order of defense layers for a nuclear reactor. The first layer of defense is the inert, ceramic quality of the uranium oxide itself. The second layer is the air tight zirconium alloy of the fuel rod. The third layer is the reactor pressure vessel made of steel more than a dozen centimeters thick. The fourth layer is the pressure resistant, air tight containment building. The fifth layer is the exclusion zone around the reactor.