Analysis of Design Basis Accidents

The safety philosophy of nuclear reactors aims at maintaining the barriers for the retention of radioactivity. The safety functions required to achieve this fundamental protection goal are implemented by the operational and safety systems. All these systems are elements in a staggered safety concept, known as the defence in depth (DiD) concept which is meant to optimize the interaction of the manifold systems in respect to their effect on the safety of the nuclear power plant. On the first two levels of the DiD concept, i.e. at normal and abnormal or disturbed operation, the operational as well as the limitation and protection systems are designed to keep the plant at normal operation or to return to normal operation after a disturbance had occurred. Only in case of a failure of these systems or by effect of external events, an operational disturbance might develop into an accident. Accident scenarios which have been considered in the plant design are called design basis accidents. Safety systems and measures are arranged at level 3 of the DiD concept to control the design basis accidents and to bring the reactor back to a safe state.