The most effective means of describing the function of each of these passive safety systems is to relate their operation in response to a LOCA. As shown in Fig. VI-6, the GE divides the LOCA transient into three regions: 1) blowdown period, 2) GDCS period, and 3) long term PCCS period.

Throughout overall LOCA transient and ECCS operation, the long term core decay heat is removed in three steps. First, the GDCS injects water into the RPV, removing core energy by boiling and venting steam into the DW through DPVs, which remain open once activated. Heat is removed from the core by natural circulation flow within the RPV. Second, the PCCS transfers energy from the DW to the PCCS/ICS pools by condensing steam from the DW in the PCCS condensers. Third, the PCCS/ICS pools transfer their energy to the atmosphere outside the containment by vaporizing pool water and venting it. The PCCS also feeds the condensate to the GDCS pools and vents noncondensable gases to the SP which enhances condensation in the PCCS pools.

There are two natural circulation mechanisms during a LOCA event. One natural circulation mechanism is natural circulation inside the RPV. Once the core is shut down, the natural circulation flow is established inside the RPV to cool down the core. The other natural circulation mechanism is natural circulation through the RPV, DW, PCCS and GDCS. The steam ejected to the DW, is condensed in the PCCS and returned to the RPV via the GDCS.