This work has been especially focused on R&D issues related to innovative components and/or systems aimed at enhancing the safety features of nuclear reactor designs. The present work, aimed at the development of safety systems to be applied to nuclear reactors, has been mainly focused on systems characterized by an intensive use of passive devices. The activation and operation of passive safety systems are generally characterized by limited driving forces (e.g. temperature difference, density gradient, pressure drop, etc.). As a consequence, the design of passive systems is more sensitive to the system parameters. The passive system design is much more complicated since the driving force, which allows the system to be activated and/or to operate, has a limited intensity, comparable with other natural forces present into the system itself. The design of a passive system requires an optimization of the geometry and an accurate evaluation of the timing to be sure that the system complies with the needed safety requirements. The main effort, that has been done in the present research work, has been mainly focused on the identification of design solutions able to improve the plant safety features. Studies have been mainly focused on passive safety systems characterized by special features that allow it to limit the possible initiating failure events that would make the system unavailable when requested. Better solutions are usually characterized by very simplified systems (number of components as low as possible) coupled with very robust designs in which the main purpose is the minimization of the forces that could counterbalance the main driving force.
The present study shows how, the proposed approach concerning the safety issues of nuclear reactors, thanks to the accidental condition prevention considered from the design stage, would make possible to avoid or to minimize the occurrence of accidental conditions. The philosophy that has been followed in the present work is that of the "safety by design"; criterion according to which the extreme simplification of the systems together with the use of natural laws to guarantee the safety functions make the whole system cheaper and safer
