In a nuclear reactor, this term is applied to containment systems where measures are taken to minimize the containment pressure in the unlikely event of rapture of any of the reactor pipes. In a water reactor, such a rupture results in ejection of water and steam accompanied by flashing of the water as the reactor and containment pressures come to equilibrium. One approach is to design the containment to be large enough for this equilibrium pressure to be kept to a reasonably low level. However containment structures contribute significantly to the overall cost of the plant and some designs, especially BWRs and integral PWRs, take steps to reduce this pressure by condensing much of the emitted steam. This can most easily be done by arranging that the path from the reactor cavity to the main body of the containment forces the steam to bubble into a large volume of water. Figure 1 illustrates the arrangement for the Advanced Boiling Water Reactor (ABWR) where the tank of water is situated in an annular region within the containment and below the level of the reactor. The water is cooled by circulating it through a residual heat exchanger, which forms the ultimate heat sink in severe emergency conditions.
Alternative arrangements in other reactors have replaced the water tank by a rack of ice trays or by racks of gravel.