There is a need to ensure a reliable confinement function for beyond design basis accidents to prevent significant release of radioactive material to the environment.

At the Fukushima Daiichi NPP, the failure of venting the containment, and the subsequent failure of the reactor building due to the hydrogen explosion, led to a significant release of radioactive material to the environment. The confinement function needs to be assessed to ensure that all possible hazards are considered in the design of equipment intended to maintain the integrity of the confinement system.