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A hermetic envelope surrounding the nuclear reactor, usually with the whole primary circuit. Its role is to protect the reactor against any damage that could be caused by external influences like hurricanes or the fall of aircraft. It also has to keep inside any radionuclides resulting from a possible nuclear accident. The containment is usually made from reinforced steel, pre-stressed concrete, or lead. Its shape varies between sphere and cylinder designs to best withstand the internal pressure and weight of the whole construction. The exact design of the containment depends on the type of reactor that it is housing and on the emergency strategies designed to eliminate the consequences of any accident. Containment is also referred to as a part of passive safety systems and as a fourth layer of protection. The first is the ceramic matrix of the fuel pellet, the second is the fuel cladding, and the third is the reactor pressure vessel.