This work explains the basic safety principles which are at the basis of the design of nuclear power plants, focusing on containemt system: the phisical barrier that separates the reactor and its components from external environment. At the beginning of the civil use of nuclear technology, containment was designed to protect and isolate external environment from the risk of an uncontrolled leakage of radioactive material from primary circuit. In the last two decades, the function of this component has been extended also to protect the nuclear facility from the risk of an external attack - natural or terroristic. Technical descriptions, drawings and schemes are given as supporting material.