1.1 Safety-by-Design™ concept

The IRIS designers used the Safety-by-Design™ philosophy from its inception in 1999. Such a designing approach had been outlined in detail in previous works (Carelli, 2005), (Carelli, 2004); here it is suffice to remember that the key idea of the Safety-by-Design™ concept is to physically eliminate the possibility of occurrence or to reduce consequences of accidents, rather than focusing only on the mitigation phase.

The most evident implication of this design approach is the choice of an integral reactor configuration, where the integral reactor vessel (containing eight internal steam generators and reactor coolant pumps) and the internal control rod drive mechanism were introduced causing the consequential absence of large primary pipes. Such a configuration enabled to have either eliminated major design basis events such as Large Break LOCA (loss-of-coolant accident) or rod ejection and also to have significantly reduced the consequences of them.

This Safety-by-Design™ approach was used by the designers of IRIS to eliminate the possibility of occurrence of certain severe accidents caused by internal events and have been extended to the external events. The focus was on the balance of plant that had not been analyzed as extensively or explicitly as NPP accidents caused by internal events. However, since extreme external events, in general, have one of the largest contributions to the degradation of the defence in depth barriers, the external events, especially for new NPPs, represent a major challenge to the designer in order to determine siting parameters and to reduce the total risk.