Allowance for severe accidents in design

Application of the defense in depth principle is obviously centered on accident prevention, i.e. provisions aimed at reducing the probability of occurrence of severe core damage. Provisions have then to be made to limit the consequences of core meltdown should it nevertheless occur. This implies definition, on the one hand, of the containment design basis accident conditions and, on the other hand, of the accident sequences which could give rise to an excessive radioactive release. In the latter case, design provisions should therefore be such as to preclude initiation of such sequences. Before presenting a certain number of provisions which could be made to cover these accident conditions (including the accident management aspects), the general radiological and probabilistic objectives set have obviously to be defined