The issue of in-vessel FCIs has been postulated in the context of present generation reactors. The defence strategies are an attempt to exclude this possibility by design or to demonstrate that the vessel will not fail or demonstrate that the containment remains intact after vessel failure. In advanced reactors, if failure of the vessel is assumed, there is the opportunity to design a reactor cavity that can survive the load (EIBL et al., 1992), and also to protect the containment from flying missiles by including an upper shield or slab.

It is generally expected that there may be a greater possibility of vessel failure if the system has been depressurised. Depressurisation is often a strategy in plants with passive injection to insure that injection can occur and so in principle in-vessel FCIs may be an issue for some advanced plant designs. However, some analysts believe that steam explosions in-vessel will not be sufficiently energetic to cause vessel failure.

The possibility of ex-vessel FCIs can be substantially reduced by preventing molten core material exuding from the bottom of the vessel from coming into contact with water. A number of preventative features are proposed in current advanced designs.

The ‘core-catcher’ has been proposed for the EPR, for example, Figure 11.3. In this design, the melt is spread horizontally over a large dry area of about 150 m2. Once in this

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Figure 11.3. European pressurised reactor. Source: Leverenz (1999).

spreading compartment, the corium would then melt through various low melting point plugs that would eventually let water through from a large IRWST tank to flood the corium (Leverenz, 1999). Heat would be dissipated from the melt by evaporation for a 0.5-1 day period, after which an alternative containment cooling system would come into operation. In the case of EPR, this involves containment sprays, cooling of the water in the spreading compartment and also cooling of the IRWST water.

Other types of core catcher have been proposed. These include a similar ‘plug melt — through’ concept into crucibles in a dry vertical core catcher concept. The crucibles are then cooled by natural circulation of water, which is ultimately discharged through the containment to an ultimate heat sink. Another type radiates heat to a large conducting surface in the reactor cavity, which is then cooled by external natural circulation.

The retention of core melt has been investigated in several experimental programmes. This includes programmes in Germany and the MACE tests in the US.

In another German design (Kuczera, 1992), the corium is allowed to fall into a dry cavity with a thin bottom layer of low melting point material. Hollow plugs are eventually uncovered allowing water to flow up the plug holes and cover the corium.

Another variation of design to achieve cooling is to have a staggered pan arrangement in an oxidic ceramic bed. The upper part of this bed remains dry and the lower part is flooded with water. Heat is extracted via natural circulation of water through the particle bed. The possibility of steam explosions is reduced because the top part of the bed remains dry.

The other way to ensure that melt does not come into contact with the water is to prevent the vessel failing. One postulated approach is to flood the vessel in the reactor cavity. The effectiveness of this measure will depend on the power density and the geometry of the vessel (surface area). In this method, heat is removed from the melt via conduction through the lower head of the vessel.