Current large LWR coolant pumps function to provide forced primary coolant flow to remove the heat generated by the fission process. Existing large PWRs utilize two to four reactor coolant pumps to provide the forced coolant flow through the primary. Natural circulation flow in current large PWRs will not provide sufficient flow to remove the heat being generated during power operation. Each 6000 to 10 000 horsepower pump provides flow of approximately 100 000 (US) gallons per minute (378,500 litres/min) to remove the heat generated by the reactor fuel assemblies for delivery to the steam generators. A driving head of approximately 90 psi (0.6 MPa) is generated by the reactor coolant pumps. A loss of flow from one or more reactor coolant pumps in a current large PWR results in a reactor trip (NRC, 2006).

These large pumps have seals to limit primary leakage and the seals require cooling. This provides a scenario for a small leak path as well as a scenario for an inter-system leakage path. Newer Generation III+ PWRs like the AP1000 design use a canned reactor coolant pump design to eliminate this possibility.

The integral nature of the iPWR pressure vessel makes the utilization of reactor coolant pumps more challenging. The two larger iPWR designs of the four active iPWR designs currently under development in the United States, the Generation mPowerTM design and the Westinghouse SMR design, plan to incorporate multiple reactor coolant pumps. Because of space limitations in the primary flow path, it is not possible to incorporate the large reactor coolant pumps used by the current PWRs. Instead, smaller reactor coolant pumps in these designs will be required to deliver the necessary flow. While specific reactor coolant pump design detail is unavailable, the resulting driving head will likely be smaller as well. Presumably, a canned type pump will be incorporated. The pumps may be located in the hot leg at the top of the steam generators or in the cold leg beneath the steam generators, depending on the design. The Generation mPowerTM design calls for 12 pumps (Kim, 2010) and the Westinghouse design calls for eight pumps (Memmott et al., 2012). In addition, the Korean SMART iPWR design also plans to use four canned reactor coolant pumps (Lee, 2010). The IRIS design planned to use eight spool pumps; one pump per steam generator (Carelli et al., 2004). The remaining two active SMR designs currently under development in the United States, the NuScale iPWR design and the Holtec SMR-160 design (a PWR-based SMR), do not plan to use reactor coolant pumps (IAEA, 2011). These designs will incorporate natural circulation cooling. In addition, the Argentine CAREM iPWR design and the Japanese IMR iPWR design plan to incorporate natural circulation coolant flow for normal operation (IAEA, 2011). All the iPWR designs will be capable of removing reactor decay heat following reactor shutdown using natural circulation cooling. Therefore, a dedicated AC backup is not required for the designs using reactor coolant pumps.