During a LOCA or transient, the control rod drive system (CRDS) shuts down the reactor. After the reactor shutdown, the main concern to safety is how to remove the decay heat. To depressurize the RPV, the ADS is actuated. As shown in Fig. VI-6, the RPV pressure decreases rapidly as soon as the ADS is actuated. The steam vented through the SRVs is sent to the SP where it is condensed, but the steam vented through DPVs goes to the DW. Large amount of steam and noncondensable gases are vented from the DW to the SP through primary horizontal and vertical vents. During this period, the SP is the primary heat sink to prevent over-pressurization of the containment and the DW pressure is adjusted by venting steam through the horizontal vents into the SP.

Since the ADS is actuated at the Level 1 signal, it is necessary to remove the decay heat in order to prevent the over-pressurization of the RPV. The ICS is designed to remove the decay heat although the RPV is at very high pressure. Similarly, the SLCS is designed to makeup the water into the RPV at high pressure before the GDCS injects water.