The four subsystems making up the passive core flooding systems connect the four core flooding pools with the headers of the four emergency condensers. These subsystems each consist of a connecting line and an integrated check valve. Normally the check valve should open when the RPV pressure drops below the pressure at the bottom of the flooding pool, and the water should flow from the flooding pool to the RPV by gravitational force alone. Under such conditions, however, only small forces are available to open the check valve. To eliminate this problem, a spring integrated into the check valve opens the valve at an RPV pressure some 2.5 bar higher than in the case of normal check valves.

When this low RPV pressure is reached, the RPV water level is lower than the inlet nozzle of the reflooding line. This means that the emergency condenser headers are filled with steam. When the spring-loaded check valve opens, steam flows from the header to the core flooding pool and is condensed there by means of a device similar to a quencher. At this point in the process the check valves serve the same function as supplementary relief valves and the pressure drop in the RPV accelerates after the check valves open. This steam blowing phase is the only phase in which natural circulation is used. The warm water of the core flooding pool flows to the water surface and the quencher is always surrounded by cold water.

Once the RPV pressure reaches the same value as that at the bottom of the core flooding pool, flow direction in the core flooding line reverses, and the water flows by gravitational force from the core flooding pool into the RPV irrespective of whether natural convection or stratification prevails in the pool or in the RPV.

The passive core flooding system was also tested at the emergency condenser test facility at Julich. Experimental results were in a good agreement with the related theoretical analyses.