The IRIS containment is inerted with nitrogen gas during operation so that the control of hydrogen concentration following postulated events and severe accident scenarios cannot cause containment pressurization due to hydrogen burn.

The IRIS is designed to provide in-vessel retention of core debris following severe accidents by assuring that the vessel is depressurized, and by cooling the outside vessel surface. The reactor vessel is cooled by containing the lower part of the vessel within a cavity that always will be flooded following any event that jeopardizes core cooling. Also, like in AP1000 [II-1], the vessel is covered with stand-off insulation, which forms an annular flow path between the insulation and the vessel outer surface. Following an accident, water from the flooded cavity fills the annular space and submerges and cools the bottom head and lower sidewalls of the vessel. A natural circulation flow path is established, with heated water and steam flowing upwards along the vessel surface, and single phase water returning downward along the outside of the vessel insulation, to the bottom of the flood-up cavity. AP1000 testing has demonstrated that this natural circulation flow is sufficient to prevent corium melt-through. Application of AP1000 conditions to the IRIS is conservative, due to the IRIS having a

much lower core power to vessel surface ratio. The design features of the containment ensure flooding of the vessel cavity region during accidents and submerging of the reactor vessel lower head in water, since the liquid effluent released through the break during a LOCA event is directed to the reactor cavity. The IRIS design also includes a provision for draining part of the water present in the PSS water tanks directly into the reactor cavity.

A diverse, passive containment cooling system is employed as part of the severe accidents management strategy, to significantly reduce the chance of containment failure.