1.3. GENERAL APPROACH

In SMR designs, as in larger reactor designs, the defence in depth strategy is used to protect the public and environment from accidental releases of radiation. Nearly all SMR designs seek to strengthen the first and subsequent levels of defence by incorporating inherent and passive safety features. Certain common characteristics of smaller reactors lend themselves to inherent and passive safety features, such as relatively smaller core sizes enabling integral coolant system layouts and larger reactor surface-to-volume ratios or lower core power densities which facilitate passive decay heat removal. Using the benefits of such features, the main goal is to eliminate or prevent, through design, as many accident initiators and accident consequences as possible. Remaining plausible accident initiators and consequences are then addressed by appropriate combinations of active and passive safety systems. The intended outcome is greater plant simplicity with high safety levels that, in turn, may allow reduced emergency requirements off-site.

It should be noted that an approach to maximize the use of inherent safety features in order to minimize the number of accident initiators in a reactor concept, and then to deal with the remaining accidents using reasonable combinations of active and passive safety systems is being pursued by the Generation IV International Forum, in line with Generation IV Technology Goals [15]. To a limited extent, such an approach is also realized in several near term designs of large capacity water cooled reactors, such as the AP1000, the ESBWR, and the VVER1000, the goal being to achieve a high level of safety in a cost effective way [4].