The reactor is operated at high pressure (7.2 MPa) which needs to be depressurized to a value closer to the containment pressure in order to gain a driving head between the GDCS tanks and the RPV. The depressurization of the RPV is achieved through the ADS during blowdown period. Once the RPV is depressurized, the GDCS injects water into the RPV without relying on any active systems by using the gravitational head between the GDCS tanks and the RPV. The GDCS functions to remove the core decay heat to avoid the core uncovering and melting. Although the PCCS is actuated during the GDCS injection period, the PCCS is not functional since there are no driving forces between the DW and WW. In other words, the GDCS water cools down the RPV and the containment pressure keeps reducing during this period.