Under low pressure conditions, elevated tanks filled with cold borated water can be used to flood the core by the force of gravity. In some designs, the volume of water in the tank is sufficiently large to flood the entire reactor cavity. As shown in Figure 3, operation of the system requires that the isolation valve be open and that the driving head of the fluid exceed the system pressure plus a small amount to overcome the cracking pressure of the check valves. The performance of the gravity drain tank may be limited under core uncovery conditions due to steam production in the core region. This is a Category D passive safety system.

Some advanced PWR designs incorporate a system to remove decay heat passively through the steam generators. This is done by condensing steam from the steam generator inside a heat exchanger submerged in a tank of water or an air cooled system as shown in Figures 4 and 5, respectively. The system shown in Figure 4 has some similar characteristics to isolation condenser. These are Category D passive safety systems.