In the initial phase of an accident with heat released to the containment, credit can usually be taken from the heat sink associated with large structures inside the containment and from the containment itself. In the longer term, these structures will equilibrate with the containment atmosphere and new heat removal systems are required to take heat away to the ultimate heat sink.

There are various types of containment design (Hochreiter, 1992). Some designs are based on steel primary containments, e. g. AP1000/600 and the APWR but others rely on concrete primary containments, e. g. SBWR (re-enforced concrete and the EPR (pre­stressed concrete). Steel containments give the benefit of good heat removal characteristics and often incorporate passive heat removal concepts. Concrete contain­ments have a proven capability to withstand greater loads but at the expense of poorer heat transfer characteristics. Concrete containments require additional heat transfer systems, e. g. heat exchangers or condenser systems to assist in heat removal from the interior to the exterior of the containment. This approach is adopted in EP 1000 (Cavicchia et al., 1997).

The end objective in extending the design basis for the mitigation of severe accidents is to limit the radiological release to the atmosphere, i. e. to reduce the source term. The obvious way to achieve this is to maintain the structural integrity of the containment, to engineer isolation of penetrations and large passages and to prevent containment by-pass sequences. If the containment function remains intact then the radiological impact will be relatively minor, certainly to the general public.

Containment performance has been segregated into different categories:

— early containment failure; this might be caused by high-pressure vessel failure and DCH, in-vessel or ex-vessel steam explosions, local or global hydrogen deflagration or possibly detonation; failures to isolate or reactivity excursions;

— late containment failure; caused by melt attack on the containment structures or pressure boundary, or long-term pressure and/or temperature increase inside the containment;

— containment bypass; interfacing LOCA, SGTR.

Some of the measures and strategies under consideration in advanced designs are discussed below. There are essentially two approaches. Either the design can be improved to withstand the loads, the loads have to be reduced or possibly a combination of both solutions can be adopted. An example of the former might be to strengthen the containment, in the latter case, a high pressure melt ejection might be avoided by earlier system depressurisation.

Measures to meet the challenges to the containment are discussed below. The phenomena relate to pressure and temperature increase associated with decay heat and gas release from a molten core, high-pressure vessel failure and DCH, steam explosions, hydrogen detonation, melt attack on the vessel pressure boundary and containment structures, and reactivity accidents (Ward, 1992).

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