**2. Passive systems implementation in advanced reactor designs**

Several advanced water cooled reactor designs incorporate passive safety systems based on natural circulation, as described in [2,3]: some of the most relevant design concepts for natural circulation systems are described hereafter and namely as regards AP600/AP1000, ESBWR and ABWR designs.

It is important to note that the incorporation of systems based on natural circulation to achieve plant safety and economic goals is being extended also to Generation-IV reactor concepts: however due to the early stage of the design - many systems are not yet established - they are not explicitly addressed.
