The WW is a large chamber with connection to the DW. During the initial blow down, the WW is directly communicated with the DW through the horizontal vents. For long term phase of the LOCA, the WW is communicated with the DW through the PCCS supply lines, condensers and the PCCS vents. When the WW pressure increases above the DW pressure, the vacuum breaker check valves are open. This action ensures that the WW is depressurized by discharging noncondensable gas from the WW to DW and the PCCS is functional. Approximately one-half of the WW volume is filled with a large volume of water that is called SP. The gas space in the WW acts as a receiver for noncondensable gases during a severe accident.

The SP plays an important role in passive safety performance because it provides: 1) A large heat sink, 2) Quenching of steam, which flows through the horizontal vents during LOCA, and 3) Scrubbing of fission products, which flow through the horizontal vents and the PCCS vents.

The WW is directly connected to the DW through twelve vertical/horizontal vent modules. Each module consists of a vertical flow steel pipe, with three horizontal vent pipes extending into the SP water. Each vent module is built into the vent wall, which separates the DW from the WW. The WW boundary is the annular region between the vent wall and the cylindrical containment wall and is bounded above by the DW diaphragm floor.

In the event of a pipe break within the DW, the increased pressure inside the DW forces a mixture of noncondensable gases, steam and water through either the PCCS or the vertical/horizontal vent pipes and into the SP where the steam is rapidly condensed. The noncondensable gases, which are transported with the steam and water, are contained in the free gas space volume of the WW. Performance of the pressure suppression in condensing steam has been demonstrated by a large number of tests.

There is sufficient water volume in the SP to provide adequate submergence over the top of the upper row of horizontal vents, as well as the PCCS return vent. When water level in the RPV reaches at one meter above the TAF, water is transferred from the pool to the RPV through the GDCS equalization lines. Water inventory of the SP and the GDCS pool is sufficient to flood the RPV to at least one meter above the TAF.