Following a LOCA, current large PWRs employ an emergency core cooling systems (ECCS) to minimize fuel damage. This is accomplished by the injection of large amounts of cool, borated water into the RCS. The ECCS also provides highly borated water to ensure the reactor remains shut down following the cooldown associated with a main steam line rupture. This water source is the refueling water storage tank (RWST), which is external to containment, except in the AP1000 PWR design. Subsystems taking suction from the RWST can include high pressure injection or charging pumps, intermediate pressure injection pumps, and the low pressure injection or RHR system. Also, containment spray pumps take suction from the RWST to limit a containment pressure spike following a LOCA or a steam line rupture in containment. Some large PWRs also employ cold leg accumulators that inject borated water into the reactor when pressure falls below the injection set point. All these systems are safety-related and are backed by the plant emergency diesel generators. Figure 5.4 shows a typical large PWR ECCS.

Similar to the Generation III+ AP1000 design, many iPWR designs have moved the RWST or its equivalent inside containment. The level of the RWST is well above the level of the top of the fuel. The iPWR designs have eliminated large — break LOCAs by eliminating all large-bore piping. Small-break LOCAs will likely not exceed the capacity of the CVCS charging pump. In addition, the small-bore piping connected to an iPWR reactor pressure vessel will be isolable as close as possible to the vessel, which will limit the probability of a small-break LOCA. In the event of a leak, the ADS valves will function to reduce pressure to the point where water can be gravity-fed from the RWST into the reactor pressure vessel to keep the core covered. All ECCS cooling is designed to be by natural circulation. As a result, the various ECCS pumps backed by emergency diesels are not required for iPWR designs.

Many iPWR designs will also use a gravity-fed boron injection tank to ensure the reactor remains subcritical following an accident. These injection tanks are located high inside the containment and serve not only as a poison source, but also provide an additional source of water for emergency decay heat removal.

The iPWR containment designs are designed to be cooled passively to limit pressure spikes following an accident. Or, the containment shell, such as in the NuScale design, is designed to accept significantly higher pressures than a current large PWR design. (NuScale, 2012) Therefore, no containment spray system is planned for the iPWR designs, except the SMART reactor (Lee, 2010).