When the integrity of the RPV is lost by core melt-through the corium has to be collected in a core catcher inside the reactor cavity where it must be cooled for the long term to prevent basemat melt-through. A typical example of this concept is the EPR design [20] where the core catcher is based on a large spreading area with provisions to avoid corium/concrete reaction and to achieve long term cooling of the molten core materials. One of the main features of this spreading concept is that, after the failure of the RPV, no immediate discharge of the corium onto the spreading area should occur. One reason for this delay is to collect the corium. The reactor cavity has to be designed to withstand the thermal and mechanical loads from the RPV failure. Therefore it has to be dry to prevent a steam explosion. Thus it should contain sufficient sacrificial material to decrease the temperature of the corium and ensure its spreadability at lower temperatures. The sacrificial material shall oxidize the remaining Zr without release of non-condensable gases (or at least with very limited amount) and bind non­volatile fission products. This delayed corium discharge will be achieved in a passive way by melt-through of a gate between the reactor cavity and the spreading area. After spreading, the corium is cooled from below by special cooling elements (similar to that used as plate condensers described above) embedded in concrete. This basemat cooling is achieved first by passive and later on by active measures. During the grace period (within 12 h after beginning of the severe accident) the spread corium is flooded from the top due to the melting of special plug-in fuses in the connection lines to the IRWST. The water evaporates at the corium surface and the condensate flows back to the IRWST. By spraying from the dome, fission products in the containment atmosphere are partly washed out.

An optional cooling concept for the corium in a spreading area utilises a spreading compartment separated by a concrete structure in a lower (spreading area) and an upper cavity [21]. Both cavities are connected by vertical channels (a riser and a downcomer). This system is flooded with the water level approximately in the middle between top and bottom of the upper cavity. Condensers at the top of the upper cavity an heat exchangers submerged in the water at the bottom of the upper cavity and in the downcomer drive a single phase NC flow for corium cooling. The cooling water flow is driven by pumps.