TABLE OF CONTENTS Severe accidents are defined as those which entail at least an initial core damage, in many cases specified as the overcoming of the regulatory fuel limits, such as a temperature of 1473 K (1200 C) in the fuel claddings, etc.). The need to consider severe accidents aside from DBAs became apparent after the final edition of the Rasmussen report was issued in 1978, when it demonstrated that core melt could have a probability (of the order of 1 in 20 000 reactor-years) which was higher than that at the time rather implicitly estimated for the then worldwide reactor list (which was roughly 500 units). This probability figure indicated an expected core melt event every 40 years on the average. Since many reactors had at that time been operating for about twenty years, the outlook was not completely reassuring. It has, however, to be considered that the same Rasmussen report envisaged that only one in about 100 core melt events could cause severe health consequences (up to 10 casualties).
In any case, the prevailing ideas of nuclear safety were not substantiated by these figures. Therefore, responsible people started to think about the best way severe accidents could be prevented, or at least mitigated. The Three Mile Island event reinforced and confirmed this need for progress in nuclear safety.
Although none of the Rasmussen report sequences replicated exactly the course of events in TMI, the report sequence TMLB was rather close to what happened there. TMI was certainly a...
