In a current large PWR, the hot RCS water leaving the top of the reactor fuel is directed into the reactor coolant system hot-leg piping. The hot-leg piping, nominally 29-inch piping (74 cm) (NRC, 2006) connects to a U-tube or a once-through steam generator, where heat is transferred from the primary water to the secondary water to generate steam to drive the turbine generator. Subsequently, colder primary water exits the steam generator into the intermediate leg (cross-over leg) piping, nominally 31-inch piping (79, cm) (NRC, 2006) which connects to the suction side of a reactor coolant pump. The intermediate leg level is typically the largest diameter pipe in the RCS and it is below the level of the top of the fuel assemblies, which can make a leak in this section of piping the most problematic of all large-break LOCAs. The reactor coolant pump discharges into the 27.5 inch (70 cm) (NRC, 2006) RCS cold leg which directs primary water back into the reactor pressure vessel. Current large PWRs employ two, three, or four reactor coolant loops. A typical two-loop large PWR reactor coolant system is shown in Figure 5.3.

Since the pressurizer and steam generator functions are integrated into the iPWR pressure vessel, all the large diameter piping associated with current large PWRs is eliminated. This eliminates any possibility of a large-break LOCA in all the iPWR designs. As a result, active emergency equipment, such as high pressure injection pumps, associated with current large PWRs to mitigate the consequences of a large — break LOCA are also eliminated in the iPWR designs.