The rods are put together into an assembly called a fuel bundle . Several hundred fuel bundles are placed into a reactor pressure vessel before the fission process takes place. The reactor pressure vessel is located in a containment building made of thick concrete and steel walls. In a reactor incident, the temperature in the pellets may raise to a point where the fuel melts. Temperature or other factors may cause the rod to crack.
Cracks allow fission products to leak into the reactor vessel.
If there is a leak in the reactor pressure vessel, the fission products may leak into the containment. If a leak in the containment occurs or if the nuclear plant operators vent the containment a plume of radioactivity may escape into the environment. The first protective barrier is the reactor fuel cladding or the walls of the fuel rod. The second barrier are the walls of the reactor pressure vessel . The third barrier is the walls of the containment building also known as primary containment .