[en] The concept of containment of radiation products by means of several barriers in cascade is explained in detail. These barriers include the fuel itself, the zircalloy cans, the closed cooling system and the main container. The oriteria for reactor safety systems are discussed and fail-safe and redundancy methods are explained. Measurements required to initiate shutdown, and emergency cooling, are also described with special reference to scram circuits operated by gravity, by reactor pressure or by nitrogen piston actuators. Diagrams of the overall safety systems are presented and a model of the Goesgen nuclear power station is illustrated. (G.M.E.)