When the water inventory of the RPV decreases from about 400 Mg to 160 Mg, the drywell around the RPV is flooded from the core flooding pool when two valves in the flooding line are opened either actively or passively (see Fig. 7). The main aim of flooding the RPV from outside is to achieve long-term retention of even a totally molten core inside an intact RPV. Experiments are in preparation which will show that cooling of the RPV is possible with thermal flux densities far below film boiling despite the numerous nozzles at the RPV bottom.

The water flows downward, driven by gravitational force alone, and fills the space around the RPV within about 30 minutes. At first the water is subcooled, but it heats up due to the hot RPV until evaporation begins. The generated steam flows through openings in the insulation of the RPV to the containment cooling condensers. The condensate again flows downward into the core flooding pool and back to the space round the RPV.

In this case, natural circulation is very effective due to both steam generation at a low eleva­tion and condensation at a high elevation in the same circuit. Because no water is lost in this natural circulation circuit, the passive transport of decay heat takes place as long as the CCCs are filled with water on the secondary side.