In most innovative reactor designs long-term retention of a totally molten core inside the RPV is foreseen by ex-vessel cooling, provided by flooding the reactor cavity from the outside [16]. In several concepts (e. g. AP600, WWER-640) the emergency water pool is used for this purpose. The emergency pool is a containment sump that covers the RPV and the lower part of the RCS. After depletion of the emergency water sources the level in the pool exceeds the elevation of the main coolant lines, thus allowing for a post accident recirculation through the vessel in a natural circulation mode.

In the SWR-1000 the reactor cavity is flooded from the core flooding pool when the water inventory of the RPV drops to 40% [15]. The water enters the space between the vessel wall and insulation through gaps at the control rod drive penetrations and heats up to the saturation state at the vessel wall. The steam leaves this gap through windows in the insulation and will be condensed at the BC. The condensate will refill the core flooding pool closing the NC loop. Retention of the core melt within the RPV means that there can be no steam explosion in the containment or corium/concrete reaction [17]. In some designs the reactor cavity is flooded via special gravity driven systems. Other concepts utilises accumulators with gas under pressure. Passive reactor cavity flooding for core retention by ex-vessel cooling is also adopted in integral reactor design (e. g. ABV-6 [18]).

In-vessel core retention systems (IVCRS) based on in-vessel cooling of the molten core materials can prevent the RPV from failure by melt-through utilising in-vessel flooding of the molten core materials [19]. Direct contact between corium and steel must be avoided and a cooling capability must be provided. TMI-2 experience showed, that a small gap between the corium (crust) and the vessel can be sufficient for corium cooling by evaporation of water. An IVCRS for in-vessel cooling can consist of a structure artificially providing such gap (in­vessel core catcher), in combination with a special gravity driven corium flooding system. As an example for an in-vessel core catcher a steel shell with some distance from the inner surface of the lower plenum is proposed. For heat removal a mass flow rate in the order of 10 kg/s is sufficient. Alternatively, IVCRS are proposed using sacrificial material having a large heat capacity inside the lower plenum.