Older large PWRs do not directly employ an automatic depressurization system (ADS). This can make it difficult to respond to a very small LOCA, because coolant pressure can hang above the discharge pressure of the high pressure injection pumps while coolant inventory is lost. Newer large Generation Ш+ PWRs, such as the AP1000 design, utilize ADSs to allow primary pressure to be rapidly reduced following a LOCA allow low pressure injection systems or gravity-fed water sources to supply water to the reactor to keep the core covered.

Smaller iPWR designs incorporate an ADS into the various protection schemes. This automatic depressurization provides the same function as it does for the newer Generation III+ passive reactor designs. In general, the automatic depressurization is initiated by opening valves connected to the pressurizer steam space. The steam is typically sparged into a water-filled storage tank inside containment or collected directly within the containment for recycling back into the core to keep the fuel covered. The contents of the containment water storage tank located high in containment can be gravity-fed into the reactor vessel after pressure has been reduced to atmospheric pressure.

5.2.3 Relief valves

All current large PWRs include safety relief valves to protect the RCS from exceeding the design pressure. The safety valves are connected to the separate pressurizer tank steam space via 6-inch (15 cm) pipe lines (NRC, 2012a). These safety valves relieve the pressurizer relief tank.

All iPWRs have the same need for safety relief valves to protect the RCS from exceeding the design pressure. Integral PWR safety valves are connected to the pressurizer steam space at the top of the integrated reactor pressure vessel. The connecting pipe would be much smaller than the 6-inch lines on a conventional PWR design. As with the ADS valves, the steam relieved from an iPWR is typically sparged into a water-filled storage tank inside containment or released directly into the containment environment. There is no pressurizer relief tank in an iPWR design.