The next generation of nuclear power plants will incorporate design features that

will eliminate or reduce the challenges to the various containment barriers or mit­igate the consequences of failure. One example is the design features included on

the European pressurized water reactor (EPR). The EPR design includes:

• The elimination of situations where the degradation of the core occurs with the primaiy circuit still at high pressure. This is achieved by high-reliability secondaiy side-decay heat removal systems but also by means of rapid depressurization via the pressurizer relief valves.

• The elimination of direct containment heating via the depressurization facility.

• The limitation of the containment pressure increase using a dedicated spray heat removal system that can subcool the water and return the pressure to atmospheric. The containment design pressure of 7.5 bars allows 12-24 hours after the accident before it is necessary to use the spray system.

• The provision of a double-wall containment with collection of all leaks in the interwall space where a lower pressure is maintained.

• The prevention of hydrogen explosions by reducing the hydrogen concentra­tion using catalytic recombiners together with selectively placed igniters.

• Accommodation of the consequences of an instantaneous full cross section rnpture of the reactor pressure vessel at a pressure of 20 bars via careful design of the layout.

• Provision to cope with molten fuel coming from a failed pressure vessel lower head, first, without a “steam explosion” and, second, preventing interaction with the containment concrete.

This is accomplished as shown in Figure 6.2 by connecting the reactor cavity

to a dedicated molten core spreading chamber via a refractory lined melt dis­charge channel. The spreading chamber has a large area (150 m2) and is nor­mally sealed from the reactor cavity by a steel plate. This plate resists melt-through for a limited time in order to accumulate the molten fuel in the cavity. The spreading compartment is connected via pipes to the refueling water storage tank in the containment. These pipes are normally closed by fusible plugs. This ensures that the water floods the spreading chamber only after the melt has been spread over the area of the chamber.