TABLE OF CONTENTS Nuclear safety activities have always been supported by a significant research effort. In fact, the majority of accident situations studied are not directly observable as they are extremely rare or even beyond any practical possibility of happening. Therefore, these situations are reproduced in laboratories or in experimental facilities, sometimes on a large scale.
At the start, the research was mainly concentrated on reactivity accidents (SPERT and BORAX experiments in the USA). Subsequently, in the 1960s, the most studied issue was a large LOCA. After Three Mile Island, the attention moved to small LOCAs because that event, together with the results of the Rasmussen Report, highlighted their danger, and to severe accidents.
Obviously, the subjects considered by safety research have been over the years much more numerous than those listed above. The main subjects have been:
The thermal hydraulic behaviour of the plant and fuel behaviour during transients and accidents.
Reactor physics in transients and accidents.
Physical phenomena specific to severe accidents (attack of the container bottom, direct containment heating, steam explosions, production and behaviour of hydrogen, behaviour of the fission products in the form of aerosols or of gases and vapours, loading of the reactor vessel by the molten core and its coolability, coolability of the molten core outside the pressure vessel, etc.).
Problems of strength of materials (irradiation effect on the pressure vessel, behaviour of the pipes in various loading conditions, leak before break and detection of pipe leaks, steam generator problems).
Structural plant problems (strength against fluid dynamic...
