When considering natural circulation systems for severe accident mitigation, one should note that the implementation of the natural circulation systems intended to cope with DBAs can essentially reduce the probability of severe accidents. Furthermore, many systems designed to cope with DBAs can be also used for severe accident mitigation purposes. The level of knowledge of natural circulation phenomena under severe accident conditions (i. e. with a degraded core, reactor pressure vessel and other plant equipment) is much lower than the level of knowledge for conditions where the plant damage is within the design limits. Keeping in mind the above considerations, in new reactor concepts the priority in the implementation of the natural circulation systems is given to the systems intended to prevent severe accidents. As for the mitigation systems, their implementation is mainly aimed to ensure that the integrity of the containment is maintained, because it is the last barrier against the release of radioactivity to the environment.

Safety systems for the mitigation of severe accident are designed to meet level 4 of the defence in depth strategy against significant releases of radioactivity to the environment. From this point of view, the main goals of severe accident mitigation are as follows:

(a) Prevention of H2-detonation,

(b) Energy removal from the containment,

(c) Corium retention inside the RPV by in-vessel and/or ex-vessel cooling, or

(d) Corium retention inside the containment by spreading and cooling of the molten core.

According to the above goals, systems for severe accident mitigation can be classified by function: H2-control, heat removal and corium/debris retention. Below the passive systems for severe accident mitigation in new designs are discussed in the context of their use of natural circulation as a design principle.