The GDCS provides emergency core cooling after events that threaten the reactor coolant inventory. Following confirmed RPV water Level 1 signal, the ADS depressurizes the RPV to allow the GDCS injection. Once the reactor is depressurized, the GDCS is capable of injecting large volumes of water into the depressurized RPV to keep the core covered for at least 72 hours following loss of coolant accident (LOCA).

The GDCS requires no external AC electrical power source or operator intervention. The cooling water flows from the GDCS pool to the RPV through simple and passive hydrostatic head. A schematic of the GDCS injection is shown in Figure VI-3. The actual GDCS flow delivered to the RPV is a function of the differential pressure between the reactor and the GDCS injection nozzles, as well as the loss of head due to inventory drained from the GDCS pool. As shown in Figure VI-3, the GDCS can be considered as two separate systems: a short term safety system and long term safety system.

The short term safety system is designed to provide short term water makeup to the reactor vessel for maintaining water level higher than the top of active fuel (TAF). There are four identical GDCS drain lines. Each GDCS drain line consists of a pipe exiting from the GDCS pool and squib valves. For short term cooling requirement, each line takes suction from three independent GDCS pools positioned in the upper elevations of the containment. In the ESBWR, there are two GDCS tanks with 602 m3 and one GDCS tank with 795 m3. Flow through each drain line is controlled by squib valves, which remain open after initial actuation. Each short term GDCS drain line connects to two GDCS injection nozzles on the RPV.

In case of a pipe break of GDCS lines at lower RPV elevations, large amounts of GDCS water can be lost and water inventory can be reduced close to the TAF. To compensate aforementioned loss of coolant in the GDCS lines, the long term cooling water is provided by a second GDCS subsystem fed by the SP. This is called GDCS equalization system. This GDCS equalization lines are opened after a prescribed time delay so that the short term GDCS pools have time to drain to the RPV and the signal of the initial RPV water inventory reduction as a result of the blowdown does not make the equalizing lines open. For long term event, the GDCS equalization lines are open when the RPV coolant level decreases to 1 m above the TAF. The squib valves are actuated in each of four GDCS equalizing lines, which connect to one RPV injection nozzle per line. The open equalizing lines leading from the SP to the RPV make long term coolant makeup possible. As shown in Fig. VI-3, GDCS equalization line nozzles are placed at a lower elevation on the RPV than those of the short term GDCS, so the GDCS equalization lines make it possible to prevent the core uncovery even though the short term GDCS injection fails. This long term GDCS equalization system also functions through purely passive hydrostatic head differences.

The GDCS pools are placed above the RPV with their air space connected to the DW. This connection effectively increases the DW air space and provides a larger volume for the released gases produced during a severe accident. After the GDCS pools are drained, the total volume of the GDCS pools are added to the volume of the DW air space.