Even if the fuel cladding (first barrier) fails, there are two more barriers to prevent a release to the atmosphere.

The second barrier designed to prevent the inadvertent release of fission products to the environment is the primary coolant system. One part of the primary coolant system, the reactor core, is located within a pressure vessel which has walls of steel up to 10 inches (25 cm) thick. This pressure vessel is called the reactor vessel. The primary coolant system contains, within its large pipes, reactor cooling water and any radioactive materials which may be present.

Failure of the reactor vessel or the associated primary coolant system piping would result in the release to the containment building (the third fission product barrier) of any fission products released from the fuel.